Hybrid-electric aircraft, and methods, apparatus and systems for facilitating same

ABSTRACT

This invention relates to electrically-driven propulsors for regional aircraft. These combine fans optimized for quiet regional operations with rotating variable pitch mechanisms contained within the hubs and highly efficient ring electric motors. The ring motors are used to drive annular fans at the inner or outer radius, or a conventional fan at the inner radius. The assembly combines a single fan, fixed or variable pitch, with a row of stators, or multiple counter-rotating fans without stators. Our focus is on propulsors optimized for quiet and efficient regional flights, but many elements apply to long-haul air and several other applications referred to herein.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Application No. PCT/US2018/023189, filed Mar. 19, 2018, which claims the benefit of U.S. Provisional Patent Application No. 62/473,446 entitled Methods, Apparatus and Systems for Facilitating Implementation of Hybrid-Electric Aircraft and Vertical and/or Short Take-Off and Landing (VTOL/VSTOL) Aircraft, filed on Mar. 19, 2017, the entire disclosures of each of which is incorporated herein by reference.

BACKGROUND

As illustrated in FIG. 1, the large fraction of commercial aircraft flights are less than 1,500 miles; in particular, more than 90% of high-traffic scheduled flights are less than 1,500 nautical miles (nm) (see, G. K. W. Kenway et al, Reducing Aviation's Environmental Impact Through Large Aircraft For Short Ranges, 48th AIAA Aerospace Sciences Meeting, 2010). This phenomenon extends to business jets as well; for example, nearly 70% of Gulfstream G450 domestic flights in the US in 2011 were to stages under 1,150 miles (Table 1).

TABLE 1 Percent cumulative distribution of US operations of a sample regional jet (E190), single aisle airliner (A320) and business jet (G450), 2011. Nm E190 A320 G450 750 65 36 57 1,000 86 58 69

In spite of the foregoing, a majority of conventional commercial aircraft are designed for considerably longer flight ranges, even though large fractions of their flights are on stages less than 1,500 miles. In particular, driven by the range-independent performance of the gas turbines used in these aircraft, conventional commercial aircraft are designed for optimal performance at long ranges, typically over 3,500 miles. The higher maximum take-off weight and cruise speed requirements given aircraft designed for longer ranges translate to a higher operating empty weight, and in turn, to a higher induced drag and fuel burn. As a result, as illustrated in FIG. 2, as much as 50% of global greenhouse gas emissions (“fuel burn”) generated by conventional commercial aircraft arises from sub-1,500 mile flight stages.

The foregoing illustrates that, in the global mobilization to mitigate the planetary threat of climate change, aviation has long been a prominent outlier. In view of the foregoing, in 2016 the National Academies of Sciences, Engineering and Medicine published recommendations on a 30-year national research agenda to reduce emissions from commercial aviation (see “Commercial Aircraft Propulsion and Energy Systems Research: Reducing Global Carbon Emissions,” National Academies of Sciences, Engineering and Medicine, 2016, hereafter “the NASEM report”).

The NASEM report was prepared by a committee of leading experts based on request by the National Aeronautics and Space Administration (NASA) to develop a national research agenda for propulsion and energy systems research to reduce emissions from commercial aviation. The committee met in 2015 and 2016 to identify high-priority research projects that could be introduced into service over the next 10 to 30 years. Amongst its recommendations, the report focuses significantly on turboelectric propulsion systems, i.e. using electric generators to convert the mechanical energy of a gas turbine into electric energy, and electric motors to convert it back into mechanical energy for propulsion). The report states that “turboelectric propulsion systems are likely the only approach for developing electric propulsion systems for a single-aisle passenger aircraft that is feasible in the time frame considered by this study.” The report also states that “turboelectric propulsion systems, in concert with distributed propulsion and boundary layer ingestion, have the potential to ultimately reduce fuel burn up to 20 percent or more compared to the current state of the art for large commercial aircraft.” And that “the committee is not aware of any system studies showing that hybrid systems would reduce emissions more than turboelectric systems.”

SUMMARY

In direct contrast to the recommendations and findings of the NASEM report, the Inventors have recognized and appreciated that hybrid-electric aircraft (which may be operated in an all-electric regime) provide viable alternatives to aircraft employing turboelectric propulsion systems, so as to realize a significant reduction in emissions from commercial aviation. More specifically, various examples of the inventive methods, apparatus and systems described in detail below relate to a hybrid-electric single-aisle aircraft that achieves significant reduction of emissions compared to conventional commercial aircraft on a majority of representative flights—and even greater emissions reductions than those postulated for aircraft employing turboelectric propulsion systems (see APPENDIX A of this application).

In other work (see APPENDIX B of this application), the Inventors described various inventive air transportation systems, apparatuses, and methods based in part on aircraft that employ a hybrid-electric powertrain. In various aspects, these systems, apparatuses and methods involved a forward-compatible (also referred to as “future-proofed”), range-optimized aircraft design that enables an earlier impact of electric-based air travel services as the overall transportation system and associated technologies are developed. Other inventive aspects included platforms for the semi-automated optimization and control of the hybrid-electric powertrain, and for the semi-automated optimization of determining flight paths for hybrid-electric aircraft, with particular examples based on regional or “short-haul” flight stages (e.g., flight distances up to approximately 1000 miles).

The present disclosure provides inventive details of aircraft designs not only for short-haul or regional flight stages, but also for medium-haul flight stages (e.g., from approximately 1000 miles to 3500 miles) and considerations for designs suitable for long-haul flight stages (e.g., greater than 3500 miles), based in part on the inventive principles disclosed in APPENDIX B. The present disclosure also contemplates further application of the inventive principles disclosed in APPENDIX B to vertical and/or short take-off and landing (VTOL/VSTOL) aircraft. In various aspects, the inventive methods, apparatus and systems disclosed herein in part rely on the “range-optimized” and “forward-compatible” concepts disclosed in APPENDIX B as applied to medium-haul, long-haul, and VTOL/VSTOL use-cases. In other aspects, the inventive methods, apparatus and system disclosed herein in part rely on example electric propulsor designs as set forth in APPENDIX B and APPENDIX C of this application.

It should be appreciated that all combinations of the foregoing concepts and additional concepts discussed in greater detail below (provided such concepts are not mutually inconsistent) are contemplated as being part of the inventive subject matter disclosed herein. It should also be appreciated that terminology explicitly employed herein that also may appear in any disclosure incorporated by reference or in the appendices attached hereto should be accorded a meaning most consistent with the particular concepts disclosed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The skilled artisan will understand that the figures, described herein, are for illustration purposes only. It is to be understood that in some instances various aspects of the invention may be shown exaggerated or enlarged to facilitate an understanding of the invention. In the drawings, like reference characters generally refer to like features, functionally similar and/or structurally similar elements throughout the various figures. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the teachings. The drawings are not intended to limit the scope of the present teachings in any way.

FIG. 1 shows cumulative seat miles by range from 2009 travel data.

FIG. 2 shows cumulative fuel burn by stage length for global aviation.

FIG. 3 is a graph illustrating a growing “disruptive range” of flight stage distances as a function of time, for which short-haul and medium-haul hybrid-electric aircraft according to the inventive principles disclosed herein are applicable.

FIG. 4 illustrates an example of a 100-seat hybrid-electric aircraft according to the inventive principles disclosed herein.

FIGS. 5A and 5B compares the performance of hybrid-electric aircraft according to the inventive principles disclosed herein with a “best-in-class” conventional B737 aircraft and a conventional Q400 regional turboprop aircraft.

FIG. 6 illustrates an example of a hybrid powertrain architecture with two gas turbine generators for the hybrid-electric aircraft of FIG. 4, according to the inventive principles disclosed herein.

FIG. 7 illustrates an example of a hybrid powertrain architecture with one gas turbine generator for the hybrid-electric aircraft of FIG. 4, according to the inventive principles disclosed herein.

FIG. 8 illustrates an example of a VSTOL aircraft according to the inventive principles disclosed herein.

FIG. 9 is an isometric view with cut-away of the quiet electric propulsor fan, duct, and motor assembly.

Appendix A contains FIGS. 10-17.

FIG. 10 illustrates the distribution of stages for single aisle and regional jets.

FIG. 11 illustrates the fuel burn by stage length for global aviation.

FIGS. 12 and 13 illustrate a “New mobility” paradigm.

FIG. 14 is an illustration of underutilized airports.

FIG. 15 is an illustration of historic long-range transportation strategy, identifying critical short-haul air routes.

FIG. 16 is a chart illustrating how transport mode varies according to trip distance.

FIG. 17 illustrates benefits of hybrid electric aircraft for regional travel.

Appendix B contains FIGS. 18-36.

FIG. 18 is a diagram illustrating certain of the primary components, elements, and processes that may be present in an implementation of an embodiment of a transportation system.

FIG. 19 is a diagram illustrating certain components, elements, data flows, and processes that may be present in an implementation of an embodiment of a transportation system.

FIG. 20 is a diagram further illustrating certain of the components, elements, and processes that may be present in an implementation of an embodiment of a transportation system.

FIG. 20(A) is a flowchart or flow diagram illustrating a process, method, operation, or function to determine recharge and refuel services required at a destination airport, and which may be used in an implementation of an embodiment of the inventive systems and methods.

FIG. 20(B) is a flowchart or flow diagram illustrating an embodiment of a process, method, operation, or function to determine recharge and refuel services en route to a destination airport.

FIG. 21 is a diagram further illustrating certain of the primary components, elements, and processes that may be present in an implementation of an embodiment of a transportation system.

FIG. 22 is a diagram illustrating an example of a range-optimized hybrid-electric aircraft that may be used in an implementation of the inventive regional air transport system.

FIG. 23 is a diagram illustrating a variable pitch electric ducted fan integrated propulsion system that may be used in an embodiment of an electric-hybrid aircraft that is part of an air transportation system.

FIG. 24 is a diagram illustrating a powertrain and its associated elements according to an embodiment that may be used in an electric-hybrid aircraft used as part of an air transportation system.

FIG. 25 is a schematic of a series hybrid drive configuration according to an embodiment for a representative aircraft that may be used in implementing a transportation system;

FIG. 26 is a diagram illustrating an example user interface for use by a pilot of an embodiment of an aircraft;

FIG. 27 is a diagram illustrating elements or modules of a powertrain optimization and control system (POCS) that, according to an embodiment may be used in an electric-hybrid aircraft that may be used as part of an air transportation system.

FIG. 28 is a diagram illustrating elements or modules of a POCS that, according to an embodiment, may be accessed and used to control or modify on-aircraft processes in an embodiment of an air transportation system.

FIG. 29 shows the interface configuration for an example powertrain coupled to the POCS onboard by several interfaces/connectors, according to an embodiment, for purposes of sensing a performance parameter and returning a control signal to a component of the powertrain or its control system.

FIG. 30 is a diagram illustrating an example flight path optimization for an aircraft that may be generated by the Flight Path Optimization Platform (FPOP) and used at least in part to control the operation of an aircraft in an embodiment of a regional air transportation system.

FIG. 31 is a flowchart or flow diagram illustrating certain of the inputs, functions, and outputs of a Flight Path Optimization Platform (the FPOP) that, according to an embodiment, may be used to determine or revise a flight path for an electric-hybrid aircraft that may be used as part of an air transportation system.

FIG. 32 is a flow chart or flow diagram illustrating an embodiment of a hybrid-electric aircraft design process that may be used in implementing an embodiment of an air transportation system.

FIG. 33 is a diagram of an example of a hybrid-electric aircraft designed in accordance with the principles and processes described herein.

FIG. 34 is a diagram illustrating the efficiency of a certain aircraft and propulsor configuration as a function of flight altitude and required power.

FIG. 35 is a diagram illustrating several regional zones and the associated airports or landing areas that may be used as part of implementing an embodiment of a regional air transportation system.

FIG. 36 is a diagram illustrating elements or components that may be present in a computer device or system configured to implement a method, process, function, or operation in accordance with an embodiment of the invention.

Appendix C contains FIGS. 37-46.

FIG. 37 is an isometric view with cut-away of the fan, duct, and motor assembly, according to.

FIG. 38 is a cut-away of the ring motor within the duct including the blade tip interface.

FIG. 39 illustrates a blade attachment at the inner hub, including a fully rotating variable pitch mechanism.

FIG. 40 is a cross section of a motor within a nacelle illustrating a minimum drag cooling system, according to an embodiment.

FIG. 41 is a cross section of a single blade defining a blade pitch angle, according to an embodiment.

FIG. 42 illustrates a blade with root and tip support, and loads acting on the fan blade, according to an embodiment.

FIG. 43 is a cut-away of an alternate bearing implementation where fan support bearings are located at the hub on the inner radius, according to an embodiment.

FIG. 44 shows an implementation of a propulsor with two counter-rotating fans and without stators.

FIGS. 45 and 46 are cross sections of a motor according to alternative embodiments.

DETAILED DESCRIPTION

Following below are more detailed descriptions of various concepts related to, and embodiments of, inventive methods, apparatus and systems for facilitating implementation of hybrid-electric (including all-electric operation) and VSTOL aircraft. It should be appreciated that various concepts introduced above and discussed in greater detail below may be implemented in any of numerous ways, as the disclosed concepts are not limited to any particular manner of implementation. Examples of specific implementations and applications are provided primarily for illustrative purposes.

Range-optimized hybrid-electric aircraft powered by a series hybrid powertrain offer several advantages to conventional commercial aircraft including, but not limited to: significantly lower operating costs, quiet and short take-off capabilities for close-in flight envelopes to communities and urban centers, significantly lower emissions and significantly greater reliability. The magnitude of the operating cost and emissions reduction is driven by the extent to which energy is sourced from energy storage units of the aircraft (e.g., battery packs) as opposed to fuel-burning energy sources (e.g., range-extending generators). To quantify the relative use of stored energy as part of the overall energy consumption of the aircraft, the “degree of hybridization” (DOH) is defined as the ratio of the stored energy used divided by the total energy required for a flight.

In various aspects, the inventive hybrid-electric aircraft disclosed herein deliver the most significant reductions in operating cost and emissions over flight stage distances where this ratio is high (>25%) and the aircraft operates in a strong hybrid-electric regime to all-electric regime (i.e., DOH=100% for flight stage distances over which the energy storage units provide 100% of the energy for the flight). Over longer flight stage distances the aircraft may be defined as operating in a mild hybrid regime (e.g., DOH<25%), with a turboelectric regime as a limiting case (e.g., in which the use of stored energy is insignificant, DOH=0).

For purposes of the present disclosure, the Inventors have defined a “disruptive range” for inventive hybrid-electric aircraft as a range of flight distance stages for which the DOH>25%. FIG. 3 is a graph illustrating a growing “disruptive range” of flight stage distances as a function of time, for which short-haul and medium-haul hybrid-electric aircraft according to the inventive principles disclosed herein are applicable. Given the rapid improvement of energy storage technologies, this disruptive range is forecast to expand over time, reaching 1,500 miles by 2035, as shown in FIG. 3. Based on this evolving disruptive range, the Inventors have contemplated a range-optimized and future-compatible hybrid-electric aircraft design process (see APPENDIX B).

When the design principles for hybrid-electric aircraft disclosed in APPENDIX B are contemplated for the lower maximum take-off weight and cruise speeds of short-haul and medium-haul flight distance stages, as opposed to the “one size fits all” paradigm of conventional aircraft design today, the impact of cost and emissions reduction is striking. For example, as discussed in greater detail below, hybrid-electric single aisle aircraft designed for intra-continental ranges (e.g., 2,200 miles US coast-to-coast) operate at 15% to 45% lower cost, 50% to 100% lower fuel burn, and 40% to 100% lower emissions over their disruptive ranges than their conventional counterparts. Given optimal speeds from Mach 0.68 to 0.72 versus Mach 0.78 for conventional airliners, flight times for these hybrid-electric aircraft are only modestly longer and at comparable altitudes. The impact for single aisle aircraft designed for shorter ranges is greater, and the impact for regional and business jets is far greater still (given the much lower efficiency of the gas turbines being replaced).

Thus, range-optimized hybrid-electric aircraft enable a significant reduction in emissions without any need for green incentives, given the appreciably lower operating costs of the hybrid-electric aircraft as compared to conventional aircraft. Moreover, these significant reductions in emissions can be extended to longer ranges via “intermediate stop operations,” whereby long flights are conducted in multiple stages with one or more recharge and refueling stops in between. Given the excess fuel carried on long flights, this equates to an added reduction in operating costs and emissions over the very significant benefits seen in the shorter and intermediate ranges.

In other examples, hybrid-electric aircraft may be designed for vertical or short take-off (VSTOL) or vertical take-off (VTOL), to achieve significantly lower operating costs, emissions, and noise, and improved reliability, relative to their conventional analogues. As discussed in greater detail below, in one example implementation forward propulsors are vectored and supplemented with stowable lift fans, ducted or otherwise, to minimize drag in cruise. The range-optimized and forward-compatible design methods then lead to appropriately-adjusted sizing of the wing and hybrid-electric powertrain versus conventional or short-take off aircraft. The range-optimized and forward-compatible hybrid-electric VSTOL or VTOL aircraft similarly offer significant operating cost reduction (e.g., 50-60% lower) and emissions reduction (over 70% lower) as compared to their conventional analogues. Moreover, the low-maintenance hybrid-electric powertrain coupled with very low fuel needs for inventive VSTOL and VTOL aircraft according to the principles disclosed herein make these aircraft well-suited for military, remote and hostile environments.

Range-Optimized, Forward-Compatible, Medium-Haul Hybrid-Electric Aircraft

With respect to the inventive principles disclosed in APPENDIX B, in exemplary implementations an innovative 3-tier range optimization process is employed to size the aircraft wing, and determine the capacity and mass of the energy storage units and the output of the range-extending generator for maximum efficiency over the prescribed ranges and speeds (e.g., see FIG. 32 of APPENDIX B and the associated text).

Applying these methods to a 100-seat medium-haul aircraft according to the inventive principles of the present disclosure, a first step of the design process is to define a 3-tier set of ranges and speeds, as shown in Table 2 below. Forward-compatible design is accomplished by defining the 3-tiers across the expected service timeline of the aircraft.

TABLE 2 Range optimization for a 100 seat airliner: 3-Tier ranges, speeds and altitudes with forward-compatibility 2025 2040 cruise cruise cruise cruise distance speed altitude distance speed altitude Range (sm) (Mach) (kft) (sm) (Mach) (ft) A electric 250 0.7 18-30 500 0.7 25-35 B disruptive 1035 0.7 27-36 1350 0.73 29-39 C extended 2200 0.66 29-33 2200 0.72 29-39

In Table 2, for each of range A (electric), B (disruptive) and C (extended), a maximum flight stage distance for the range is defined as a function of time (e.g., in 2025, and in 2040). As discussed above, the “disruptive range” represents flight stage distances for which the degree of hybridization (DOH) for the aircraft is greater than 25%. Maximum cruise speed is chosen to match the disruptive range (B); the typical mid-lift cruise speed of M 0.72 is sufficient to accomplish the majority of flights within this range in under two hours while taking advantage of the lower Mach number (as opposed to M 0.78 for conventional single aisle aircraft) to leverage state-of-the-art in natural laminar flow (NLF) on a low sweep wing, for a lighter, lower drag aircraft.

As recognized and appreciated by the Inventors, the selection of cruise altitude for the range-optimized hybrid-electric aircraft does not adhere to conventional rules of flying at the maximum available altitude to maximize efficiency. In particular, when seeking to maximize the disruptive range (e.g., increase the largest flight stage distance for which DOH>25%), cruising at lower altitudes enables either a 5-10% smaller generation engine for a fixed disruptive range requirement, or an 3-5% longer disruptive range with the same generation engine. The lower cruise altitude increases available generation power from a gas turbine generation engine with altitude-dependent lapse rate. This non-intuitive result is due to altitude-independent propulsion efficiency of the variable-pitch electrically-driven ducted fan, and the gas turbine generation reaching peak efficiency at lower altitudes than a conventional high bypass turbofan.

The innovative aircraft design process described in APPENDIX B considers an estimate of aircraft weight. As further recognized and appreciated by the Inventers, satisfactory performance of a 100-seat hybrid-electric aircraft suggests maximum takeoff weights of no more than 5-10% higher than conventional aircraft. Thus, for a 100-seat hybrid-electric aircraft according to the inventive principles disclosed herein, the maximum aircraft weight may be estimated at approximately 100,000 lbs. based on current aircraft designs.

The quiet electric propulsors are sized for the disruptive cruise speeds and altitudes given in Table 2, which results in a fan pressure ratio of 1.15-1.19. These ratios are closer to conventional high bypass turbofans, and higher than those discussed in APPENDIX B (e.g., 1.02 to 1.10). However, these pressure ratios are still lower than those for a conventional aircraft, via optimized motor-fan integration at lower rpms with variable pitch.

The methods described in APPENDIX B are used to size the electric propulsor motor-drive; this process results in lower shaft power required at the propulsor than in conventional aircraft due to the no-lapse electric drive, higher static thrust from the lower-pressure variable-pitch ducted propulsors, and thrust preservation on takeoff, which reduces balanced field length and results in three motors of 5.2 MW peak, 4.4 MW power output, each.

Continuing with the range-optimization process of APPENDIX B, simulation models are constructed, and an objective function is defined for optimization over the 3-tier ranges and speeds given in Table 2 in the following sequence:

-   -   The wing is first range-optimized for minimum required mission         energy with a takeoff and landing constraint of a 5000 ft.         runway; and     -   Stored energy mass, generation capacity, and fuel available are         sized through iterative performance simulation as described in         APPENDIX B, and the process is repeated across the range of         expected energy storage densities and generation efficiency         levels.

The resulting aircraft and powertrain characteristics are shown in FIG. 4. An outcome of the process is the energy storage mass fraction of 21.6%; this is slightly higher than the 12-20% range given in APPENDIX B, as would be expected for a longer range, higher speed aircraft.

FIGS. 5A and 5B show performance comparisons of the hybrid-electric aircraft according to the inventive principles disclosed herein as exemplified by the aircraft in FIG. 4, with a “best-in-class” conventional B737 aircraft and a conventional Q400 regional turboprop aircraft. In particular, FIG. 5A shows a comparison of well-to-wake greenhouse gas (GHG) emissions for a B737 and Q400 aircraft (dashed lines) with the 100-seat aircraft of FIG. 4 (solid lines, labeled as TZ100 HEV in FIG. 5A). The two solid lines in FIG. 5A for the 100-seat aircraft of FIG. 4 respectively presume a “U.S. Mix” fuel-generated electricity used to charge the hybrid-electric aircraft (upper solid line) and renewable-generated electricity used to charge the hybrid-electric aircraft (lower solid line); in both instances, a significant reduction in GHG emissions can be seen on either basis for the 100-seat aircraft of FIG. 4. FIG. 5B illustrates the reduction in total mechanical energy (TME) and direct operating cost (DOC) as a function of flight stage distance (range in statute miles) for the 100-seat aircraft of FIG. 4 as compared to the B737.

The aircraft as described in FIG. 4 utilizes two gas turbine generation engines as shown in the powertrain schematic of FIG. 6. However, as part of the design process, a trade can be made between two or more generation engines vs. a single, larger, and more efficient engine, as shown in FIG. 7, which requires an increase in stored energy reserves to maintain an equivalent level of safety. While the details of this trade will be specific to the aircraft type and operating environment, an example process is provided here for the 100-seat airliner in the current CFR 14 Part 25 regulatory environment.

Utilization of a single gas turbine generator requires an increase in the minimum stored energy reserve to ensure continued safe flight and landing following a failure of the generation turbine. The energy required is calculated in a two-step process: first the required battery-only flight distances are determined for each of the 3-tier ranges:

-   -   No modification to A range given no usage of generation, no         failure mode;     -   Over the B range, identify the worst case failure point; this         was found to be a critical distance prior to the top of descent         point on a flight where generation is used continuously in         cruise and batteries are nearly depleted to minimum for nominal         operations. As distance from the top of descent increases,         battery remaining increases but less quickly than energy         required; however additional airport options also increase. For         the 1035 sm range in the described embodiment, the worst case         failure point is 200 miles prior to the intended airport of         arrival, and that airport is maintained as the final         destination; and     -   Over the C range, battery reserve is less, but also depletes         more slowly. The same 200 sm range following failure is utilized         for this flight given high likelihood of alternate airport         within a 200 sm radius at any point in a flight.

In the second step, the total reserve energy required is calculated using simulation and is the sum of: a) energy required to reach the airport over a gradual drift-down profile at best range speed+b) energy to fly the runaway approach pattern and land+c) energy to execute a missed approach, circle back around, and execute a second landing to a full stop. It is assumed that the batteries will be depleted to their minimum voltage during the maneuver, below typical operations. At maximum range and generation use, additional storage reserves are a minimum of 10% above the nominally depleted pre-landing state to ensure safe completion of the flight to landing. For the aircraft in FIG. 4, this trade saves 6% in fuel burn, and requires a 21% increase in stored energy reserve. With a modular hybrid-electric powertrain, this additional reserve could be carried only on flights when required for safety.

When compared the mid-life performance of the aircraft of FIG. 4 to an emerging state of the art conventional equivalent, this combination of attributes provides: 10-35% reductions in DOC; 35-70% reductions in total mission energy; and 30-95% reductions in emissions, with low to zero emissions within airport terminal areas. As noted in APPENDIX B, these benefits continue to improve over time with continued improvements in energy storage technology.

Note that the system and methods for range optimization from APPENDIX B and applied without modification resulted in an aircraft with the attributes of a strong hybrid, including a DOH of 25% at the outer extent of the disruptive range (B).

In other aspects, the inventive hybrid-aircraft design of FIG. 4 and similar implementations may be further characterized by additional features which improve system performance including, but not limited to:

-   -   Improved aerodynamics with natural laminar flow (NLF) on a low         sweep wing;     -   Fuselage aerodynamics leverage boundary layer ingestion (BLI) by         the integrated aft propulsors, which also reduce vertical tail         size with centerline thrust;     -   The outer ring implementation of the electric propulsor         motor-drive is utilized to further reduce nacelle ram drag by         minimizing nacelle length, maximizing diffusion and BLI on the         aft fuselage with the aft most possible location;     -   BLI is further enhanced through utilization of three smaller         fans with a higher fraction of the disk immersed in the boundary         layer. As recognized by the Inventors, this is only feasible         when utilizing fully electric propulsion since conventional         turbofans may not be arranged in close proximity such due to         disk failure burst requirements;     -   Aft fuselage generation-propulsion configuration enables a         simple and flexible integration with the powertrain, minimizes         wire weights and EMI;     -   Very low wing location for wing-integrated battery packs close         to the ground for easy line access; and the landing gear is         short, minimizing the higher MLW penalty;     -   As noted in APPENDIX B, the electric fans provide precise thrust         control including regenerative braking, eliminating the need for         spoilers, reducing approach noise and system weight;     -   As noted in APPENDIX B, additional noise benefits include         airframe shielding of fan tonal noise (otherwise adversely         affected by BLI), and an exceptionally quiet cabin due to aft         propulsors and generators. In addition, the low sweep wing         eliminates the need for a landing slat, also a significant noise         source;     -   As noted in APPENDIX B, three propulsors near the centerline         provide more benign failure modes than a single wing mounted         engine, further enhanced by powertrain fault tolerance and         graceful degradation; and     -   As noted in APPENDIX B, the fan integration shielding by the         fuselage and tail surfaces results in 15 to 20 EPNdB noise         reduction below current (single observer).

Range-Optimized, Forward-Compatible, Long-Haul Aircraft

In yet other inventive implementations, the optimization is constrained by a minimum cruise speed for range (C), which results in a longer range (i.e., long-haul) aircraft with reduced savings. The modular nature of the hybrid-electric powertrain is then leveraged to increase generation size to meet the extended M 0.72 cruise, and the increased fuel requirement is offset with reduced energy storage resulting in an aircraft operating in the “mild hybrid” regime with a DOH<10% at the outer limit of the long-haul range.

Range-Optimized, Forward-Compatible VTOL Aircraft

In yet other inventive implementations, various principles disclosed in APPENDIX B are applied to range-optimize a forward-compatible Vertical Takeoff and Landing (VTOL) aircraft. The aircraft is configured for vertical flight with swiveling primary propulsion, and two supplemental, folding lift fans to minimize the cruise drag impact of the VTOL system. This configuration of only four lift points would normally result in a catastrophic loss of control following failure of any one propulsor. In one inventive implementation, the failure hazard is prevented by designing the powertrain for fault-tolerant operation with graceful degradation of thrust capability as described in APPENDIX B. Fault detection and recovery are performed in real time by the Powertrain Optimization and Control System (POCS), also as described in APPENDIX B, with the result that the powertrain may suffer a loss to any one component during hover without significant loss of thrust on any propulsor or loss of control.

The range optimization design cycle for the VTOL aircraft is conducted using the same process and methods of APPENDIX B, as described above for the 100-seat airliner. The 3-tier range, speed and altitude requirements are given in Table 3; for this application, the forecast energy storage densities by the 2050 forward compatibility date result in transition to fully electric operation so that no extended range targets are specified.

TABLE 3 Range optimization for a VTOL aircraft: 3-Tier ranges, speeds and altitudes with forward-compatibility 2030 2050 cruise cruise cruise cruise distance speed altitude distance speed altitude Range (sm) (mph) (kft) (sm) (mph) (ft) A electric 275 320 12-24 900 350 15-30 B disruptive 550 320 15-24 1500 350 15-30 C extended 750 280 15-20 n/a n/a n/a

The range-optimization process of APPENDIX B is not modified for the VTOL capability; as compared to a conventional takeoff and landing aircraft, there are two changes to the constraints imposed on the optimization space: 1) The wing sizing constraint for takeoff and landing is removed resulting in a smaller wing sized for cruise only; and 2) the very high power requirements for hover in takeoff and landing always require stored energy in addition to the generation, resulting in a stored energy emergency reserve even on hybrid systems.

The resulting aircraft is shown in FIG. 8. Comparing performance to next generation compound rotor helicopters, the range-optimized hybrid VTOL provides 50-70% reductions in DOC and 65-80% reductions in fuel burn. As noted in APPENDIX B, these benefits continue to improve over time with improvements in energy storage technology, including transition to full electric operation.

As further understood by the Inventors, the variations in configurations between VSTOL and VTOL, and Degree of Hybridization result in substantially different methods needed for the calculation of the required energy reserves and how those reserves are sourced. And further, that there is a critical distinction between air vehicles with lifting surface of sufficient size to allow a safe landing on a runway without the use of powered lift (VSTOL), as opposed to vehicles which must utilize significant powered lift to land (VTOL).

VSTOL aircraft, while capable of vertical flight, may also be configured to fly and land safely as a conventional aircraft on a standard runway. For this class of vehicle, the safety reserves may be calculated in the manner discussed in APPENDIX B as a function of generation power available:

-   -   For an aircraft in which generation power is sufficient for         sustained flight and landing, required reserve energy may be         stored entirely as fuel, allowing full depletion of stored         energy at the completion of the planned flight for maximum         efficiency. The amount of reserve fuel required is calculated as         needed to meet the regulations pertaining to the aircraft type         and class of operation, typically either a flight time or flight         profile.     -   For an aircraft in which generation power is insufficient for         sustained flight, the reserve is calculated as the sum of: a)         the fuel needed to fly the required reserve time or flight         profile, and b) the stored energy needed to supply all required         flight power in excess of the generation power for the reserve         flight time, or flight profile.

VTOL aircraft have insufficient aerodynamic lifting surfaces to allow a safe runway landing without powered lift. As is well known by those familiar with the art, a powered lift landing requires nearly maximum propulsion power output for a short, but sustained period of time, unlike a conventional runway landing flown at much lower power levels. For architectures where generation power is below required peak power, the stored energy units are utilized to provide the difference between peak and generation power. For these aircraft, the reserve is calculated as the sum of: a) the fuel needed to fly the required reserve time or flight profile, including a landing, and b) the stored energy needed to supply all required power in excess of the generation power for the reserve flight time, or flight profile, including a landing.

Quiet Electric Propulsors

APPENDIX B described quiet electric ducted fan propulsors of very low pressure ratio (1.02 to 1.10) for regional ranges and speeds. Electric ducted fans designed for higher cruise speeds and altitudes result in fan pressures ratios much closer to conventional bypass fans; 1.15-1.25 for medium-haul airline class aircraft, and as high as 1.5 for a high Mach, high altitude business aircraft applications. Any electrically-driven ducted fan propulsor of the form described in APPENDIX B, as well as those described in APPENDIX C, may be designed with the pressure ratio needed for high efficiency cruise. The resulting system embodies exemplary benefits: net propulsive efficiencies greater than 80% with energy conversion efficiency greater than 70%; 15 to 25 dB lower noise than conventional propulsion alternatives; high static thrust for short runways; ultra-low to zero terminal area emissions; no altitude lapse for quick climbs; rapid thrust response for steep approaches; regenerative braking to replace spoilers; simplified reverse thrust for landing. And improved safety and strong thrust preservation given one or multiple high peak-power drives designed for fault-tolerance and graceful degradation.

Additional details of electric propulsor designs suitable for use in the various aircraft described herein are provided in APPENDIX C.

The electric propulsor is an assembly comprised of an electric ring motor coupled with a fixed or variable pitch fan with a plurality of blades coaxially mounted inside a duct. The ring motor assembly provides the drive torque and in one implementation consists of an electric machine including a rotor and a stator, with the rotor affixed to the fan blade tips, and the stator affixed to the duct. The ring motor and fan assembly is supported by bearings at the outer or inner radius designed for quality gap, rotor to stator and fan to duct, to ensure high aerodynamic and electromechanical efficiencies. In other implementations, a ring motor is used to drive an annular or conventional fan at the inner radius.

FIG. 9 is an isometric view of one embodiment with a ring motor at the outer radius. As recognized by the inventors, maintaining the gap between motor rotor and stator 38 is critical to efficient motor operation. The construction of the rotating structural ring 22 utilizes a high modulus material, such as carbon fiber composites, which at the optimal rotating speeds for a ducted fan of 2000-4000 rpm, limits radial deformation to a fraction of the gap width. In this implementation, the rotor is supported by caged roller bearings 33 with the bearing cage attached to the fixed structure 35 and the matching race formed by the rotating motor structure 22. While this implementation uses physical bearings, magnetic, or fluidic bearings (e.g. air bearings) are also within scope of the invention.

The motor stator (windings or magnets) 31 are affixed to the primary support structure 35 through the periodic cross-tie members 39. The fixed structural rings 35 provide the attach points for integrating the motor with airframe. As recognized by the inventors, the concentration of masses and loads along the rotational plane of the fan, supported by rings with high structural moments of inertia, results in a very light, structurally efficient propulsor unit.

The fan blades may be actuated to vary the blade pitch angle to the oncoming flow in flight. Blade connections to the outer propulsion ring, and inner hub ring are made such that the blade may rotate around its longitudinal axis (fan radial direction). A mechanism within the hub including a drive motor and a system of gears rotates all fan blades simultaneously.

Ring motor designs offer higher power densities and greater opportunities for lightweight air cooling. They also translate to a much stiffer structure than conventional center mounted configurations, delivering higher propulsive efficiencies without the weight penalty of the stiffening otherwise required to maintain a small tip gap. In the case of a ring motor driving an annular fan at the outer radius, high quality tip gaps are maintained via bearings, mechanical, air, magnetic or other at the outer or inner radius. The lower stiffness of a ring motor driving at the inner radius is still significantly higher than that of a conventional center mounted drive. Additional benefits include significantly reduced duct and stator structures given much lower mass suspended within the duct, fan and stators less compromised by structural requirements, and simpler rotating variable pitch mechanisms contained within the radius of the ring motor.

In one implementation, a single propulsion fan is followed by a fixed stator coaxially located within the duct. In another implementation, multiple propulsion fans are located sequentially within the duct, and which alternate in direction of rotation. In a third implementation, the electric ring drive motors are utilized to drive an open propeller, or high speed open rotor.

Additional Features

APPENDIX B described a variety of inventive methods, apparatus and systems (collectively “inventive principles”) that extend to the aircraft examples described herein.

One such inventive principle is the regional air transport network which may include 4-classes of airports, most with runways >1,500 ft (or pads for VTOL aircraft), and differentiated based on their respective role in the regional network and the degree to which they are equipped to support high-frequency hybrid-electric flights: Regional tier I, II and III airports, and Mainline large hubs in-region. Like conventional airliners, the large (>90 seats) range-optimized hybrid to all-electric aircraft are more likely to operate to the mainline hubs and regional tier I airports that have runway and other services to support the volume of passengers. As described in APPENDIX B, these would be equipped with relatively fast recharge and swap stations tailored for the needs of the larger aircraft. The mainline hubs together with regional tier I airports accessible by large hybrid to all-electric aircraft collectively define a continental air system that can be served by the aircraft via direct or intermediate stop operations. Range optimized strong hybrid to-all electric VSTOL aircraft, on the other hand, will access the entire range of airports via runways or VTOL pads.

Other inventive principles that may apply to the inventive aircraft examples disclosed herein include the energy storage units, comprised of battery packs, supercapacitors, or other media for storing electrical energy (or a combination thereof), coupled with battery management systems that manage operation and safety of the packs. Each pack may comprise of multiple individually removable battery modules, and operate either with some or all of these modules in place.

Yet other inventive principles that may apply to the inventive aircraft examples disclosed herein include the powertrain optimization and control system (POCS), the Flight Path Optimization Platform (FPOP) and the Recharge and Refuel Platform (RRP).

APPENDIX A—COMMERCIALLY VIABLE REDUCTION OF AVIATION CARBON EMISSIONS Abstract

In the global mobilization to mitigate the planetary threat of climate change, aviation has long been a prominent outlier. But that is slowly starting to change. Responding to a request by NASA, the National Academies of Sciences, Engineering and Medicine recently published recommendations on a 30-year national research agenda to reduce emissions from commercial aviation. We offer a contrary and much more hopeful view. Up to 40% of aviation emissions can be eliminated by 2035 by building on technologies drawn from ground electric vehicles. Powerfully, the reduction comes packaged with a scale lift in long-distance mobility, aligning vast commercial and individual interests with the planetary goal. Delivering on this critical opportunity will require reset and acceleration of development priorities, given potential for impact starting early 2020s.

Context

Early in October, the International Civil Aviation Organization (ICAO) general assembly agreed to carbon-neutral growth for international aviation, requiring all CO₂ growth beyond 2020 levels to be offset by investments in environmental projects elsewhere. This historic agreement followed attempts over two decades to include aviation in UN climate efforts post-Kyoto. In another recent milestone, the Environmental Protection Agency declared in July that jet engine exhaust endangers public health. This ruling paves the way for the agency to regulate aviation to achieve US goals under the Paris Climate Agreement: reducing emissions by 28 percent below 2005 levels by 2025. The international and US milestones follow action in 2012 by the EU to include flights within the European Economic Area under the EU Emissions Trading Scheme (ETS).

In the meantime, aviation emissions have been growing at an average of 4.3 percent per year since 1999 and today account for 4.9 percent of cumulative climate change impact of human activities. Given continued growth, aviation emissions are expected to triple by 2050, taking up a quarter of the global emissions budget at that time.

Although the ICAO agreement was widely welcomed as long overdue, the offsetting proposal remains controversial. Given that innovation lags the rapid growth of the sector, most expect the airlines to meet the bulk of their commitments through offsets “the quickest to implement, the easiest to administer, and the most cost-efficient.”¹ Critics fear that any pressure on the sector to decarbonize will be insignificant, and worse still, that the environmental benefit of offsets will be lost in the opacity and double-counting that bedevils these today. ¹ Reducing emissions from aviation through carbon neutral growth from 2020, Working Paper developed for the 38th ICAO Assembly, 2013.

Against this backdrop, the National Aeronautics and Space Administration (NASA) requested the National Academies of Sciences, Engineering and Medicine to convene a committee to develop a national research agenda for propulsion and energy systems research to reduce emissions from commercial aviation. The committee met in 2015 and 2016 to identify high-priority research projects that could be introduced into service over the next 10 to 30 years. Its findings were recently published in the report “Commercial Aircraft Propulsion and Energy Systems Research: Reducing Global Carbon Emissions.”

The report focuses on propulsion and energy technologies applicable to large commercial aircraft, single-aisle and twin-aisle, because such aircraft account for more than 90 percent of global emissions today. Based on this, the committee recommended focus on four areas and identified four others for reduced prioritization (Table 4 below).

TABLE 4 Recommended focus of research to reduce aviation emissions High-priority Low-priority Aircraft-propulsion integration Hybrid-electric² propulsion systems Gas turbine engine improvements High-power batteries Turboelectric propulsion systems SOFC and PEM fuel cells Sustainable alternative jet fuels Superconducting motors, generators

Our response is driven by the flawed premise that forms the basis for this report: that the single-aisles and twin-aisles of today will define the workhorse platforms of the future. In cost-driven sectors such as commercial aviation, such a static view is appropriate only if the technologies at play do not fundamentally alter the underlying cost structure. Witness the current reshaping of the electric utilities by the arrival of solar and wind generation. As a consequence, the recommendations in the committee report far short of what can be done to respond to climate change, and what we as an advanced nation should aspire to achieve over a 30-year timeframe. In the following, the Inventors describe an alternate perspective. ² We use the term “hybrid-electric” for the class of charge depleting hybrid and all-electric systems.

Findings

40 percent of aviation emissions (4% of transportation) are generated over sub-1,000 mile stages and can be largely eliminated by 2035 via charge depleting electrification technologies derived from ground electric vehicles.

Despite the 70-year shift of the airlines to larger aircraft and longer ranges, the vast majority of departures today are to destinations less than 1,000 miles away. These departures dominate regional jet and single-aisle operations. In 2011, for instance, 58 percent of A320 and 85 percent of E190 flights in the US were of less than 1,000 miles (FIG. 10 Error! Reference source not found.).³ As a result, even though large commercial airliners account for 90 percent of aviation emissions, short-haul stages of under 1,000 miles generate a sizeable 40 percent of the total (FIG. 11).⁴ ³ An economic analysis of maximum take-off weight reductions under an ICAO CO₂ standard, GRA, 2013.⁴ The Aero2k project: Aviation emissions inventory, Eurocontrol, 2002.

This detail is crucial because emissions reduction technologies and their effectiveness vary significantly by range and energy intensity of the transport application. On the ground, impact has ranged from disruptive, ultra-low to zero emissions, over shorter ranges via charge depleting vehicles, e.g., cars, transit buses, delivery trucks, to modest over longer ranges via charge sustaining platforms, e.g., heavy-duty trucks. As a result, shorter range vehicles are electrifying quickly, with immediate impact on emissions, while the pace of deployment over longer ranges is much slower. Moreover, it is the rapid electrification of shorter range vehicles that has powered the development of technologies that are beginning to appear on long-range platforms. For example, the electrification of medium-to-heavy duty trucks builds largely on technologies developed for transit buses and light-duty trucks.

The Inventors have recognized and appreciated that the aviation will split along similar lines. Short-haul emissions can be largely eradicated in the near-term by charge depleting technologies derived from ground vehicles, while long-haul emissions require alternate strategies for much more modest reductions over time (Table 5). Ranges impacted by charge depleting aircraft are a function of the energy density of storage and the power density of low-emissions generators. Based on the current stable of technologies, breakthrough impact will extend to 1,000 mile ranges by the mid-2020s, reducing emissions by 50 percent or more. Continued development over the decade that follows will stretch this range to 1,500 miles, covering 80 percent of single-aisle stages. Given the recent pace of development and the potential for wildcards to emerge, this outlook will very likely be exceeded.

TABLE 5 Range-driven split of emissions reduction technologies for aviation. Regional Long-haul Hybrid-electric propulsion Hybrid, turboelectric propulsion Aircraft-propulsion integration High-power batteries, including redox flow Gas turbine generators Fuel cell generators Wireless power transfer Sustainable alternative jet fuels

Replicating the sequence on the ground, regional aircraft powered by charge depleting technologies will emerge in the early 2020s to lead their long-haul analogues by a decade or more. This will create a cost- and risk-effective development path, from less powerful, lower and slower regional platforms, to the more powerful, higher and faster long-haul. Many aspects of the hybrid-electric propulsion systems for the regionals carry over to the turboelectrics that are a focus of propulsion research today, reducing the incremental development required.

This suggests a research agenda that traverses regional and long-haul technologies, along with staging to reflect their sequential entry in the 2020s and 2030s. Focus should extend beyond turboelectrics powered by gas turbines. Given the 10 to 30 year timeframe of interest, one or more alternate technologies are very likely to become competitive as primary or secondary sources of propulsive power. Aviation would be better served by an aggressive pursuit of promising alternates, and by hybrid-electric platforms that are engineered to accommodate them.

Growth of electrified aviation will be powered by the rapid development of energy storage and electric drive technologies, on track to far exceed the expectations of the sector.

Unprecedented investment across personal electronics, ground electric vehicles, smart grids, renewables and grid storage are driving the rapid development of key component technologies, such as batteries, supercapacitors, electric drives, power electronics and fuel cells. And the pace will only accelerate as deployment scales up around the world. Demand for rechargeable batteries, for instance, is forecast to almost triple from 62 GWh in 2015 to 164 GWh in 2025.

Unfortunately, aviation today is impeded by conservatism, mirroring the behavior of other sectors when first impacted by electrification. Studies have often overestimated what component performance is needed for use on aircraft or underestimated how well state-of-the-art components perform. The report, for instance, refers to electric motors of specific weight in the range 1.5 and 2 kW/kg, although as much as 13 kW/kg could be achieved in the near term (TRL 3 in under 2 years) powered by controllers of 19 to 25 kW/kg. On batteries, the report speaks to specific energy in the 200 to 250 Wh/kg range, although several commercially available chemistries exceed 300 Wh/kg today, and Tesla is expected to exceed 400 Wh/kg within the year. In fact, the 400 to 600 Wh/kg projected over 30 years is likely to be exceeded within 10 given aggressive pursuit of 500+Wh/kg chemistries by well-resourced programs around the world.

This conservatism has led to timelines that are 10 to 15 years behind component technologies. The report states that “turboelectric propulsion systems are likely the only approach for developing electric propulsion systems for a single-aisle passenger aircraft that is feasible in the time frame considered by this study.” The report also states that “turboelectric propulsion systems, in concert with distributed propulsion and boundary layer ingestion, have the potential to ultimately reduce fuel burn up to 20 percent or more compared to the current state of the art for large commercial aircraft.” And that “the committee is not aware of any system studies showing that hybrid systems would reduce emissions more than turboelectric systems.” In direct contrast, we find that hybrid single-aisle aircraft powered by 600 Wh/kg batteries, available by 2025, could reduce emissions by almost 50 percent on a majority of representative flights.

FIG. 12 is an illustration of “New mobility” to 1,000+ miles across ground and air for breakthrough door-to-door times and fares. The impact of charge depleting technologies will extend far beyond aviation emissions as hybrid-electric air will emerge as a fast and flexible regional transit mode, addressing an additional 4 to 8 percent of transportation emissions by driving 7 to 15 percent of long-distance ground trips to air.

The powerful forces of sharing, electrification, autonomy and intermodal connectivity are driving an epic transformation of ground transport. Charge depleting aviation technologies will take this “new mobility” revolution to the skies, with impact extending from 600 miles in 2020, to over 1,000 miles by 2030. Hybrid-electric aircraft, coupled with new mobility on the ground, will enable door-to-door travel at breakthrough economics and speeds, 2 to 5 times better than highways, high-speed rail or conventional air (FIGS. 12 and 13). The revolution will come with much lower emissions, much lower community noise, and require minimal infrastructure beyond airside recharge or swap. The mode will light up a vast stock of underutilized airports across much of the world⁵ (FIG. 14 Error! Reference source not found.) to address a vast regional transport gap.⁶ ⁵ Country Comparison: Airports, The World Factbook, CIA, 2013.⁶ Gabriel Bouladon, The transport ‘gaps’, Science Journal, 3, No. 4, 1967.

The tremendous value of such regional air has long been recognized. In the late 1960s, as part of the first long-range transportation strategy for the US, the newly formed Department of Transportation (DOT) identified short-haul air (FIG. 15) along with high-speed rail as modes required to keep US infrastructure current.⁷ “We envision a short-haul system capable of operating from every runway and urban area V/STOL port in the United States and very comfortable in an environmental sense,” said John Shaffer, FAA Administrator, in 1970.⁸ To achieve this, an $800 million Federal program was launched to develop a short-haul air system, across the aerospace OEMs, leading airlines, DOT and NASA. This envisaged continental US to be served by 7 short-haul zones. While long-haul would connect hubs across zones, short-haul would serve point-to-point traffic within a zone and feeder traffic to the hubs. ⁷ Final report to the Aviation Advisory Commission: Short-haul air transportation study, Mitre, 1972.⁸ Statement before the Subcommittee on Aviation of the Senate Committee of Commerce, US DOT, 1971.

The program was halted in the turmoil of the 1973 oil crisis. Several more modest efforts followed, such as the Quiet STOL Aircraft and Small Transport Aircraft programs of the 1970s and 1980s, respectively. In the 2000s, the EU Small Aircraft Transport program identified regional air as key to improved mobility, complementing conventional air and high-speed rail.⁹ However, all of these were doomed by the lack of cost-competitive propulsion for regional aircraft. As a result, what Fred Bradley, a leading aircraft financier at Citibank, observed in 1980 remains true today, “there is a large market out there . . . and we've been approached practically daily on financing for this group of carriers. But the basic problem is that there really isn't an economic vehicle that will permit carriers to operate profitably.”¹⁰ ⁹ Small Aircraft Transport as a new component of the EU Air Transport System, SAT Roadmap, 2011.¹⁰ Senior Vice President at Citibank, Proceedings of OTA Advanced Air Transport Advisory Panel, 1980.

By overturning the cost structure of gas turbines, regional air powered by charge depleting hybrid-electric aircraft will have far reaching impact on mobility and community development. The 70-year shift of the airlines to larger aircraft, longer ranges, and a shrinking set of high-volume hubs has created a “transport gap” over regional ranges where there is no real alternative to highways. Roughly 95 percent of long-distance trips in the US are sub-1,000 miles, and air today serves fewer than 4 percent of these (equates to 14 percent of passenger miles, FIG. 16).¹¹ The withdrawal of the airlines to longer routes has left communities increasingly disconnected from the global air network, with adverse impact on their local economies and ability to attract employment.¹² The Federal government is engaged via the Essential Air Services (EAS) program, but given six-fold increase from 1993 to $232 million in 2013, Congress is looking to cap growth. High-speed rail, the only other option for fast regional travel, is limited by high infrastructure costs to a few very high-density corridors with passenger volumes large enough to support the investment. ¹¹ America on the Go, Findings from the National Household Travel Survey, BTS, US DOT, 2006.¹² Aviation: Status of Air Service to Small Communities and Federal Programs Involved, US GAO, 2014.

Studies in the US and the EU indicate that the mode could shift between 7 and 15 percent of long-distance trips from ground to much faster regional air. Replacing ground vehicles with ultra-low to zero emissions aircraft would reduce transportation emissions by a further 4 to 8 percent. Development of the mode will also come with additional environmental benefit. Air quality around major airports will improve due to zero-emissions operation of hybrid-electric aircraft on the ground, and the shift of a significant fraction of regional traffic to secondary and tertiary airports. Development of hybrid-electric propulsion systems in the 2020s will accelerate the electrification of long-haul aircraft in the 2030s. And under the ICAO agreement, regional hybrid-electric flights would qualify as carbon offset for their long-haul brethren.

Implications

The conservatism is harmful in several ways. It translates to a delayed and modest response to the threat of climate change, forgoes a once-in-a-generation window to leapfrog to leadership in high-speed regional transport and electrified aviation, and leaves US aerospace vulnerable to intensifying competition from overseas. The conservatism is also at odds with what we as a nation can achieve over a 30-year timeframe. In roughly half that time, NASA intends to fly a manned mission to Mars, while JAXA will beam a gigawatt of power from orbiting solar satellites to the ground.¹³ ¹³ Japan demoes wireless power transmission for space-based solar farms, IEEE Spectrum, 2015.

Instead, the scale of the threat requires an equally bold response that is both, comprehensive over the long-term, and aggressive in the near-term. In the long-term, the response must focus on reducing aviation emissions to match that of a sustainable benchmark, such as that of electricity sourced from the grid. Large state-of-the-art gas turbines on long-range cruise are indeed competitive with scale power plants today. However, the transition of the grid to a largely renewables basis over the timeframe of interest will leave them far behind, creating need for sustainable alternatives. The alternate fuels prioritized by the report are one such option. These should be supplemented by other promising candidates so the sector is not at the mercy of a single class of solutions. Breakthroughs could arrive from a range of areas under active development today, such as high power batteries including redox flow, fuel cells and wireless power transfer. The recommendation to deprioritize sources of power other than gas turbines is a barrier to innovation. Instead, effective alternatives should be sought out and championed.

Emissions generated on sub-1,000 mile stages are a prime target for aggressive near-term reduction. As outlined previously, 8 to 12 percent¹⁴ of transportation emissions could be targeted by 2035 via charge depleting technologies derived from ground electric vehicles. The key to rapid progress on this front, with impact extending over time to long-haul, is an embrace of the new mobility revolution. ¹⁴ 4 percent (regional aviation) plus 4 to 8 percent (shift of 7 to 15 percent of long-distance ground to air).

With vast markets at stake, the battle for mobility on the ground is delivering change that was unimaginable only a few years ago. Battery pack costs have plummeted from $600 per kWh in 2012 to $200 to $250 per kWh today (Tesla at $150 to $200 per kWh), on track to $120 per kWh in the early 2020s for parity with conventional vehicles. The DOT recently announced the “most comprehensive national automated vehicle policy that the world has ever seen” last month, autonomous taxis are being tested in Singapore and Pittsburgh, and major automakers have announced plans for fleets of on-demand electrified autonomous cars. So much so that “widespread adoption of battery-powered vehicles is a serious threat to the oil industry” driving need to prepare for “radical change” given real risk of an “investor death spiral” as capital flees the sector.¹⁵ ¹⁵ Oil groups ‘threatened’ by electric cars, Financial Times, 2016.

As the battle extends to longer ranges, aviation will be similarly reshaped. As a foreshadowing of this, consider the well-funded ventures attacking intra-urban vertical lift. These include Zee Aero and Kitty Hawk funded by Larry Page¹⁶, the Vahana program at Airbus¹⁷, Ehang in China, to declared interest at Uber. Each of these is developing all-electric and semi- or fully-autonomous aircraft (FIG. 12). On-demand fleets of such aircraft are destined for the major metros in the early 2020s, offering commuters a zero emissions escape from the traffic below. Given breakthrough performance and costs, impact is likely to far exceed that of aircraft and helicopters flying such operations today, with significant emissions reduction benefit. ¹⁶ Welcome to Larry Page's Secret Flying-Car Factories, Bloomberg, 2016.¹⁷ Airbus aims to be first to certify self-flying air taxi, Aviation Week, 2016.

Several factors suggest that once hybrid-electric aircraft emerge (FIG. 17), they will quickly dominate regional air. Adoption of ground electric vehicles was impeded by higher costs, but as these approach parity, growth is accelerating. In contrast, hybrid-electric aircraft will undercut the conventional right from the start, over ranges extending from 600 miles in 2020 to 1,000 miles by 2030. Scale-out will be driven by the much greater convenience of regional travel, at door-to-door times and fares 2 to 5 times lower than the best today. Given fast regional legs to hubs, long-haul journeys will speed up as well, lifting mobility nationwide to place goals such as “4-hour door-to-door anywhere” in the EU¹⁸ within reach. ¹⁸ Flightpath 2050, Europe's Vision for Aviation, European Commission, 2011.

In turn, the scale-out of regional air will mature electrification technologies and supporting infrastructure to levels near those required for long-haul aircraft. Increasingly capable regional hybrid-electrics of the 2020s will extend to hybrid or turboelectric long-haul airliners of the 2030s with moderate capital and risk. In contrast, the proposed one-step to single-aisle turboelectrics, with power levels in the 20 to 25 MW range, is fraught with risk for relatively modest emissions reduction. As a result, resource levels committed by the aerospace majors to electrified long-haul airliners remain low, underlining an uncertain future, driven largely by oil prices, regulation and government support. It is estimated that carbon allowances under EU ETS would need to be priced over 10 times higher “before having any real influence on fleet planning or the acquisition and further development of new aircraft technologies.”¹⁹ Moreover, the 2030s entry would delay climate action by two decades, and forgo the opportunity to defray development costs and risk via electrified regional platforms. ¹⁹ EU ETS and the aviation industry: Between a rock and a hard place, CAPA Centre for Aviation, 2011.

Recommendations

Accordingly, we recommend a national research agenda that balances priorities across two timeframes. To deliver aggressive emissions reduction over sub-1,000 mile ranges in the near-term, and to establish a comprehensive path to reduce long-haul emissions to a sustainable benchmark in the long-term. The latter including a portfolio of low-emissions technologies with potential to emerge as a viable source of primary or secondary power over a 30-year timeframe (Table 5).

Crucially, the research agenda must be staged to pace what will be a market-driven journey of the sector into a low-carbon future. This can be accomplished by targeting research at increasingly demanding technologies, aligned with the sequence by which increasingly powerful aircraft will be commercialized. We believe the 2020s will be dominated by sub-1,000 mile hybrid-electric platforms that will take new mobility to the air. Driven by plummeting costs of batteries, hybrid-electrics will displace gas turbines over these ranges. As a result, aviation emissions will decline, passengers and carriers will become familiar with electric propulsion, technologies and infrastructure will mature. The success of hybrid-electrics on regional flights will pave the way for scale-up to the long-haul hybrids and turboelectrics of the 2030s, by driving demand, relieving risk and reducing resource needs.

We also believe the agenda is best directed at aspirational emissions and mobility goals enabled by electrified aviation. In fact, breakthrough mobility is the highest leverage pathway to reduced emissions, given it will galvanize vast commercial and public interests towards the planetary goal:

-   -   Zero emissions air transport to 1,000 miles by 2035, extending         to 1,500 miles by 2045, delivering additional emissions         reduction by shift of long-distance trips from ground to air.         Commercial aviation emissions to exceed a sustainable benchmark         by 2045.     -   Fast and flexible door-to-door mobility everywhere     -   Door-to-door costs and times of regional trips reduced to half         in 2035, and to a third²⁰ by 2045. Retirement of EAS subsidies         by 2030.     -   Same day, round trip access by air anywhere in the US²¹ by 2045,         similar to the EU 4-hour door-to-door time target for the         region.     -   US leadership of ultra-low emissions power for aviation, via         focus on a portfolio of viable technologies such as gas         turbines, sustainable alternative fuels, high-power batteries         including redox flow, fuel cells and wireless power transfer.     -   US leadership of new mobility in the air, including quick-access         vertical and short take-off airfields, all-weather GPS         navigation to airfields, air traffic control for high-volume         regional flights, sustainable micro-grids for airfields,         autonomous and remotely-piloted aircraft technologies. ²⁰ Based         on reduction of door-to-door times by factors between 2 and 5 on         individual trips.²¹ Follows the NASA Aeronautical Technology         Theme, Travel baseline definition door-to-door time study,         John A. Volpe National Transportation Systems Center, 2003.

Electrified propulsion opens the door to a new era of aviation, with impact likely to rival the 70-year transformation of global travel by gas turbines. The most potent path to a green future is one that engages its revolutionary promise.

APPENDIX B—PCT PUBLICATION NO. WO 2016/093905—“SYSTEM AND METHODS FOR IMPLEMENTING REGIONAL AIR TRANSIT NETWORK USING HYBRID ELECTRIC AIRCRAFT” Background

Transportation devices and transportation systems are an important part of the infrastructure used to enable commerce and the movement of people between locations. As such, they are essential services for the growth of an economy, the development of a society, and the effective governance of a region. Transportation devices and systems are used to move goods between distributions points, enable face-to-face meetings and discussions, and in general to facilitate the growth of relationships. Further, as new modes of transportation have developed, travel times and cargo carrying abilities have changed drastically, enabling new and often faster methods of communications and the delivery of goods and services. In this regard, over the years, several primary types of transportation systems have been developed; however, each typically has its own focus, advantages, and drawbacks compared to other modes of transportation.

For example, in the United States today, over 100 years after the first powered flight, the vast majority (>97%) of regional long-distance trips (i.e., 50 to 500 miles) are made by personal auto. Although countries with extensive rail systems may divert 10-15% of trips to rail, this still leaves well over 80% of trips to be made by auto. This is inefficient and may also not be in the best interests of society at large, as it translates to poor mobility (relatively long door-door times), creates pollution, and puts stress on the existing highway infrastructure. However, current commercial air services over this range are often relatively costly and inconvenient. One reason for this inefficiency is that the shorter flight distances mean that a relatively large fraction of the total travel time (>70%) is spent on the ground (where this “ground” time includes traveling to and from airports, traversing terminals, at the gate or taxiing on the tarmac). As a result, in such situations, air transportation is generally not a desirable mode of transport and is currently used for less than 1% of such regional trips.

Aviation transport services for people and cargo have doubled approximately every 15 years, enabling unprecedented global mobility and cargo distribution. In contrast, the relatively poor value proposition (and hence usage) of air travel over regional ranges might be considered a striking failure; even more so, given that almost all (94%) long-distance travel is regional. In this sense there is a demonstrated need for a desirable form of regional distance air transportation, but a lack of a desirable system for satisfying that need.

This failure to develop an effective and efficient form of regional air transportation has led to stagnant door-to-door travel times and has been a significant factor in limiting mobility improvements in the United States for several decades. This is highly undesirable, as limited mobility impacts business and pleasure travel, job development and opportunities, educational choices, and other factors which are beneficial to the growth and prosperity of society. In some regards, the viability of regional air transportation has actually declined steadily since the 1960s as airlines have shifted to larger aircraft and longer ranges in order to respond to competitive pressures and to lower the cost-per-passenger-mile of transportation. Thus, the present economic forces are causing current methods of providing air transportation to move steadily away from the types of systems and methods described herein.

As will be described, conventional approaches to providing air transport services for regional travel are not sufficiently convenient or effective for purposes of encouraging widespread use by potential customers. Embodiments of the invention are directed toward solving these and other problems individually and collectively.

SUMMARY

The terms “invention,” “the invention,” “this invention” and “the present invention” as used herein are intended to refer broadly to all of the subject matter described in this document and to the claims. Statements containing these terms should be understood not to limit the subject matter described herein or to limit the meaning or scope of the claims. Embodiments of the invention covered by this patent are defined by the claims and not by this summary. This summary is a high-level overview of various aspects of the invention and introduces some of the concepts that are further described in the Detailed Description section below. This summary is not intended to identify key, required, or essential features of the claimed subject matter, nor is it intended to be used in isolation to determine the scope of the claimed subject matter. The subject matter should be understood by reference to appropriate portions of the entire specification of this patent, to any or all drawings, and to each claim.

As recognized by the inventors, the failure of modern aviation services to address the need for regional air transport is a direct outcome of the use of conventional aircraft technology. It is well known by those experienced in the art that optimizing conventional aircraft for regional operations results in design and performance compromises which adversely affect efficiency. For example, gas turbines (jet and turboprop engines) suffer a significant decrease in efficiency at lower altitudes and slower speeds, and a further loss of efficiency when scaled to smaller sizes. In addition, short runway operations impose a penalty in wing and/or engine sizing larger than optimal for efficient cruise performance. As a result, large aircraft over long-ranges offer the lowest operating costs per passenger-mile, with rapidly increasing costs for distances <500 miles, and for aircraft seating fewer than 100 passengers (or an equivalent cargo weight of 25,000 lbs). Note that given a relatively poor efficiency on the ground or in climbing mode, scaled down gas turbines cost more to operate at short-ranges relative to longer ranges (where for shorter ranges, ground or climbing time may represent a significant and relatively larger percentage of the overall travel time).

This inefficient cost relationship shapes many of the aspects of aviation services today. Competitive pressures have driven airlines to migrate to larger aircraft and longer flights. This has led to fewer flights from a smaller number of hub airports that can generate passenger volumes sufficient to support the larger aircraft. For example, the United States has approximately 13,500 airports; yet, 70% of the air traffic is concentrated in 29 hubs and 96% is concentrated in 138 hubs. Fewer flights from a small number of increasingly congested hubs, coupled with long ground transit times have in turn caused the relatively low utility of air transportation for purposes of regional travel. Further, the recent, heavier focus on “capacity discipline” by the airlines has exacerbated the problem as airlines seek to concentrate demand to even fewer hubs.

Embodiments of the invention are directed to systems, apparatuses, and methods for overcoming the disadvantages of current air transportation systems that might be used for regional travel by providing a more cost effective and convenient regional air transport system. In some embodiments, the inventive air transport system, operational methods, and associated aircraft include one or more of the following elements, functionality, or features:

-   1. A highly efficient plug-in series hybrid-electric powertrain,     specifically optimized for aircraft operating in regional ranges; -   2. A forward compatible, range-optimized aircraft design, enabling     an earlier impact of electric-based air travel services as the     overall transportation system and associated technologies are     developed; and -   3. Platforms for the semi-automated optimization and control of the     powertrain, and for the semi-automated optimization of determining     the flight path for a regional distance hybrid-electric aircraft     flight.

In one embodiment, the invention is directed to a hybrid-electric aircraft, wherein the aircraft includes:

-   -   a source of energy, the source of energy including a source of         stored electrical energy and a source of generated energy         provided by a generator;     -   a powertrain, the powertrain operable to receive as an input         energy from the source of energy and in response to operate one         or more electrically powered motors;     -   one or more propulsors, wherein each propulsor is coupled to at         least one of the one or more electrically powered motors;     -   an electronic processor programmed with a first set of         instructions, which when executed provide one or more functions         or processes for managing the operation of the aircraft, wherein         these functions or processes include a function or process for         determining a status of the amount of stored electrical energy         and generator fuel presently available to the aircraft;     -   determining an amount of stored electrical energy and generator         fuel required to enable the aircraft to reach its intended         destination;     -   determining an amount of energy that could be generated by the         source of generated energy presently available to the aircraft;     -   determining how to optimally draw energy from the sources of         stored electrical energy and generated energy; and     -   in event of failure or abnormal operation of a component of the         powertrain, determining a reconfiguration of the powertrain and         a revised control strategy for continued flight;     -   an electronic processor programmed with a second set of         instructions, which when executed provide one or more functions         or processes for planning a flight for the aircraft, wherein         these functions or processes include a function or process for     -   accessing data regarding the total amount of stored electrical         energy and generator fuel presently available to the aircraft;     -   determining if the amount of stored electrical energy and         generator fuel presently available to the aircraft is sufficient         to enable the aircraft to reach its intended destination,         wherein this includes consideration of a first aircraft         operating mode wherein stored electrical energy is used         exclusively and consideration of a second aircraft operating         mode wherein a combination of stored electrical energy and         generated energy is used;     -   if the amount of stored electrical energy and generator fuel         presently available to the aircraft is sufficient to enable the         aircraft to reach its intended destination, then planning a         route to the intended destination;     -   if the amount of stored electrical energy and generator fuel         presently available to the aircraft is sufficient to enable the         aircraft to reach its intended destination, then planning how to         optimally draw energy from the sources of stored electrical         energy and generated energy over the planned route to the         intended destination;     -   if the amount of stored electrical energy and generator fuel         presently available to the aircraft is insufficient to enable         the aircraft to reach its intended destination, then planning a         route to an intermediate destination, wherein planning a route         to an intermediate destination further includes     -   determining one or more possible energy and/or fuel providers;     -   determining if available stored energy and generator fuel are         sufficient to reach at least one of the providers;     -   generating a route to the at least one provider; and     -   planning how to optimally draw energy over the route; and     -   a communications element or elements operable to enable data         from the aircraft to be transferred to a remote data processing         platform or operator and to receive data from the remote data         processing platform or operator for exchanging data regarding         one or more of route planning or recharge and refuel sources.

In another embodiment, the invention is directed to a regional air transportation system that includes a plurality of the inventive hybrid-electric aircraft, a plurality of aircraft take-off or landing sites, wherein each take-off or landing site includes a recharge and refuel platform operable to provide recharging services for a source of stored electrical energy and fuel for a source of generated energy, and a data processing system or platform, wherein the data processing system or platform is operable to provide route planning data to one or more of the plurality of hybrid-electric powered aircraft.

In yet another embodiment, the invention is directed to a non-transitory computer readable medium on which are contained a set of instructions, wherein when executed by a programmed electronic processing element, the set of instructions cause an apparatus containing the electronic processing element to:

-   -   determine a status of the amount of stored electrical energy and         generator fuel presently available to a hybrid-electric powered         aircraft;     -   determine an amount of stored electrical energy and generator         fuel required to enable the hybrid-electric powered aircraft to         reach its intended destination;     -   determine an amount of energy that could be generated by a         source of generated energy presently available to the         hybrid-electric powered aircraft;     -   determine how to optimally draw energy from the sources of         stored electrical energy and generated energy; and     -   in event of failure or abnormal operation of a component in the         powertrain, determine a reconfiguration of the powertrain, and a         revised control strategy for continued flight.

Other objects and advantages of the present invention will be apparent to one of ordinary skill in the art upon review of the detailed description of the present invention and the included figures.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention in accordance with the present disclosure will be described with reference to the drawings, in which:

FIG. 18 is a diagram illustrating certain of the primary components, elements, and processes that may be present in an implementation of an embodiment of the inventive transportation system 100;

FIG. 19 is a diagram illustrating certain of the primary components, elements, data flows, and processes that may be present in an implementation of an embodiment of the inventive transportation system 200;

FIG. 20 is a diagram further illustrating certain of the primary components, elements, and processes that may be present in an implementation of an embodiment of the inventive transportation system 300;

FIG. 20(A) is a flowchart or flow diagram illustrating a process, method, operation, or function to determine recharge and refuel services required at a destination airport, and which may be used in an implementation of an embodiment of the inventive systems and methods; FIG. 20(B) is a flowchart or flow diagram illustrating a process, method, operation, or function to determine recharge and refuel services en route to a destination airport, and which may be used in an implementation of an embodiment of the inventive systems and methods;

FIG. 21 is a diagram further illustrating certain of the primary components, elements, and processes that may be present in an implementation of an embodiment of the inventive transportation system 400;

FIG. 22 is a diagram illustrating an example of the inventive range-optimized hybrid-electric aircraft 500 that may be used in an implementation of the inventive regional air transport system;

FIG. 23 is a diagram illustrating a variable pitch electric ducted fan integrated propulsion system 600 that may be used in an embodiment of an electric-hybrid aircraft that is part of the inventive air transportation system;

FIG. 24 is a diagram illustrating a powertrain 700 and its associated elements that may be used in an embodiment of an electric-hybrid aircraft used as part of the inventive air transportation system;

FIG. 25 is a schematic of a series hybrid drive configuration 800 for a representative aircraft that may be used in implementing an embodiment of the inventive transportation system;

FIG. 26 is a diagram illustrating an example user interface 900 for use by a pilot of an embodiment of the inventive aircraft;

FIG. 27 is a diagram illustrating the primary functional elements or modules of a powertrain optimization and control system (POCS) that may be used in an embodiment of an electric-hybrid aircraft that may be used as part of the inventive air transportation system;

FIG. 28 is a diagram illustrating the primary functional elements or modules of a POCS that may be accessed and used to control or modify on-aircraft processes in an embodiment of the inventive air transportation system;

FIG. 29 shows the interface configuration for an example powertrain 1200 coupled to the POCS onboard by several interfaces/connectors 1202 for purposes of sensing a performance parameter and returning a control signal to a component of the powertrain or its control system;

FIG. 30 is a diagram illustrating an example flight path optimization for an aircraft that may be generated by the Flight Path Optimization Platform (FPOP) and used at least in part to control the operation of the aircraft in an embodiment of the inventive regional air transportation system;

FIG. 31 is a flowchart or flow diagram illustrating certain of the inputs, functions, and outputs of a Flight Path Optimization Platform (the FPOP) that may be used to determine or revise a flight path for an electric-hybrid aircraft that may be used as part of the inventive air transportation system;

FIG. 32 is a flow chart or flow diagram illustrating a hybrid-electric aircraft design process that may be used in implementing an embodiment of the inventive air transportation system;

FIG. 33 is a diagram of an example of a hybrid-electric aircraft designed in accordance with the principles and processes described herein;

FIG. 34 is a diagram illustrating the efficiency of a certain aircraft and propulsor configuration as a function of flight altitude and required power;

FIG. 35 is a diagram illustrating several regional zones and the associated airports or landing areas that may be used as part of implementing an embodiment of the inventive regional air transportation system; and

FIG. 36 is a diagram illustrating elements or components that may be present in a computer device or system 1900 configured to implement a method, process, function, or operation in accordance with an embodiment of the invention.

Note that the same numbers are used throughout the disclosure and figures to reference like components and features.

DETAILED DESCRIPTION

The subject matter of embodiments of the present invention is described here with specificity to meet statutory requirements, but this description is not necessarily intended to limit the scope of the claims. The claimed subject matter may be embodied in other ways, may include different elements or steps, and may be used in conjunction with other existing or future technologies. This description should not be interpreted as implying any particular order or arrangement among or between various steps or elements except when the order of individual steps or arrangement of elements is explicitly described.

Embodiments of the invention will be described more fully hereinafter with reference to the accompanying drawings, which form a part hereof, and which show, by way of illustration, exemplary embodiments by which the invention may be practiced. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy the statutory requirements and convey the scope of the invention to those skilled in the art.

Among other things, the present invention may be embodied in whole or in part as a system, as one or more methods, as one or more elements of an aircraft or transportation system, as one or more elements or functional modules of an aircraft (flight) control system or regional aircraft transportation system control system, or as one or more devices. Embodiments of the invention may take the form of a hardware implemented embodiment, a software implemented embodiment, or an embodiment combining software and hardware aspects. For example, in some embodiments, one or more of the operations, functions, processes, or methods described herein for use in the flight control (or other form of control) of an aircraft or of a transportation system may be implemented by one or more suitable processing elements (such as a processor, microprocessor, CPU, controller, etc.) that is part of a client device, server, or other form of computing or data processing device/platform and that is programmed with a set of executable instructions (e.g., software instructions), where the instructions may be stored in a suitable data storage element. In some embodiments, one or more of the operations, functions, processes, or methods described herein may be implemented by a specialized form of hardware, such as a programmable gate array, application specific integrated circuit (ASIC), or the like. The following detailed description is, therefore, not to be taken in a limiting sense.

Prior to describing multiple embodiments of the inventive aircraft and associated regional air transport network, it is noted that the following acronyms or terms may be used herein, and are meant to have at least the indicated meaning with regards to concepts, processes, or elements:

-   -   ADS-B: Automatic Dependent Surveillance-Broadcast—the air-to-air         and air-to-ground communication and data which allow NextGen air         traffic control.     -   ATC: Air Traffic Control—refers to both the controller, and the         flight path assigned to the aircraft.     -   BPF: Blade Passage Frequency, in Hz for a ducted fan. Calculated         as rotational frequency (Hz) divided by number of blades.     -   Conventional aircraft engine: combustion engines currently in         use to provide aircraft propulsion, including, but not limited         to, reciprocating or rotary internal combustion engines, gas         turbines, turboprops, turbojets, turbofans, and ram jets.     -   COT: Cost Of Time—in this context refers to the cost of time for         the passengers or payload. For example, a business jet assigns a         very high cost of time for their passengers, while cargo has a         much lower COT. A measure of the “value” (and hence a factor in         the pricing) of an amount of time to a particular passenger, for         a piece of cargo, etc.     -   DOC: Direct Operating Cost, calculated as the sum of energy         (fuel, and/or electricity), energy storage unit amortization,         and maintenance reserves for airframe and range extending         generators or engines.     -   Ducted Fan: A multi-bladed aerodynamic propulsor located in an         axial flow duct. The duct is shaped to maximize the efficiency         of the fan.     -   FMS: Flight Management System, an integrated computer system         which controls an aircraft through an auto-pilot and         auto-throttle interface. The FMS is typically programmed prior         to take off and can fly the aircraft without pilot intervention         much, or all, of the way to the destination.     -   I: Indirect costs of operation on an hourly basis, including         airframe depreciation, crew costs, insurance, etc.     -   Mach number: The fraction of the speed of sound that a vehicle         is moving.     -   Range extending generator: May be comprised of internal         combustion engines, each driving one or more motor-generators;         alternately, could be comprised of units that convert stored         chemical energy directly to electricity, e.g., hydrogen fuel         cells.     -   Rechargeable Energy Storage Unit: comprised of battery packs,         supercapacitors, or other media for storing electrical energy         (or a combination thereof), coupled with a battery management         system(s) that manages operation and safety of the packs. Each         pack may comprise of multiple individually removable battery         modules, and operate either with some or all of these modules in         place. Also referred to as an “Energy storage unit”.     -   Solidity: measure of area of the propeller disk occupied by         blades. Defined as the ratio of total blade chord at a given         radius to the circumference of the fan disk at that radius.     -   STOL: Short Take Off and Landing—not a rigid definition, but         implies significantly shorter runway lengths, and steeper         approach angles than a similarly sized, non-STOL equipped         aircraft.     -   TDI: Turbo Diesel Injection—a compression ignition engine with         boosted intake manifold pressure.

In some embodiments, the inventive transportation network may be defined by airports (and associated ground transport options), aircraft, and demand-supply mechanisms optimized for regional electric air transport services. This combination of technologies, processes, devices, and control methods may be used to provide multiple benefits to users. Regional electric air transport offers significantly lower door-door travel times and cost per mile than alternate travel modes: highways, high-speed rail, and conventional air. As a result, the inventive system will drive and support four large-scale applications:

A. Scheduled commercial: Regional electric air will be capable of offering twice the door-to-door speed of conventional air at approximately half the fare, along with convenience and comfort. Unlike the highly concentrated air network of today, large aircraft flying long ranges to a declining set of high-volume hubs, the inventive regional electric air network will be (much) more distributed. Smaller aircraft flying lower will serve a large number of community airports. The greater choice of schedules and destinations, along with low-traffic routes will result in a far more personalized travel experience than obtained from air travel today. Regional electric air will serve two major pools of demand: point-to-point and feeder. Point-to-point flights will serve destination pairs within a region, typically bypassing conventional aircraft and hub airports. Feeder flights will transport passengers from their local regional airports to more distant conventional hubs to connect to a long-haul flight out of the region. Conversely, feeders will transport passengers arriving on long-haul flights to their local regional airport. Both will dramatically reduce door-to-door travel times for regional as well as long-haul travel, by bypassing congested hubs and by reducing ground legs;

B. Business and on-demand: The value proposition of a regional electric air transportation system for business and on-demand travel is also a strong one. Electric aircraft offer comfortable travel over regional ranges at 80 to 90 percent lower costs than business jets. In addition, quiet STOL (short takeoff and landing) capabilities will open up all-hours access to a large pool of smaller airports, offering door-to-door times comparable to faster business jets, which require longer runways and create noise pollution and other problems. Moreover, the disruptively low costs of electric air transportation will expand demand for this form of travel, while sharing technologies will multiply usage options. In addition to air taxi, charter, and fractional ownership modes available today, capacity may also be offered on a shared or on-demand basis. For example, on shared flights, open seats on existing flights will be offered to other passengers often at reduced fares. On-demand flights, on the other hand, will be scheduled based on passenger volumes. These will include an on-demand marketplace that will accept passenger requests for flights, enabling flights to be scheduled based on a combination of requests and historical demand patterns;

C. Cargo: Even as the regional transport infrastructure has stagnated over the past decades, the demand for fast delivery of goods has multiplied, driven by the rapid growth of online commerce. Electric air transportation will offer a disruptive alternative, offering door-door speeds 4 to 5 times faster than ground, at comparable or lower cost. This will be enabled via cargo flights (manned, remotely piloted or autonomous) from airports at or near regional logistics hubs to airports at or near local depots. As example, fast delivery of goods to a home or business will be enabled by electric air transport cargo flights between regional distribution centers and local supply depots. Electric aircraft will be loaded at the distribution centers with packages routed to one or multiple local supply depots. Once loaded, aircraft will take-off from adjacent or nearby airstrips for regional flights to airstrips adjacent to or near each of the local supply depots to which cargo is addressed. Delivery from the local depot to the final destination may use an existing mode, e.g., delivery truck, or one of several emerging platforms, e.g., autonomous vehicle, delivery drone. As another example, fast delivery of goods to point of use will be enabled by electric air transport flights from the corresponding point of production (e.g., manufacturing facility, farm) or logistical hub (e.g., warehouse, transport terminal). Electric aircraft will be loaded at the point of production or logistical hub, take off from nearby airstrips for fast flights to airstrips near the point of use;

D. Military: Despite tremendous advances in military technology over the past decades, development of platforms to transport troops or cargo over regional distances has largely stagnated, and remain limited largely to ground convoys, or the much less cost effective conventional aircraft or rotorcraft. In much the same way as for cargo, electric aircraft could transform regional military logistics, by enabling the shift of a fraction of supply convoys from ground to electric aircraft. Doing so would reduce exposure to enemy action, increase supply chain velocity by a significant factor (estimated to be a factor of 5 or greater), at costs comparable to or below that of ground transport. As an example, fast supply of forward bases could be enabled by electric air transport flights from theatre logistical hubs. Electric aircraft could be loaded at a logistical hub with troops and cargo routed to one or multiple forward bases. Once loaded, aircraft will take off from a nearby airstrip for regional flights to airstrips near each of the forward bases addressed. Delivery could also be made without touchdown at the forward bases, using parachutes or other mechanisms to direct the cargo to the base safely. Other opportunities include replacing conventional aircraft or rotorcraft on tactical transport missions for faster travel, increased stealth, and significantly lower cost; and

E. Manned and un-manned: Given the rapid and continued development of autonomous vehicles and remotely piloted drones, the four applications of regional electric air transport services described above may include conventionally piloted aircraft, as well as aircraft that are designed with increasing degrees of autonomy. These will include piloted aircraft equipped for back-up control by a remote pilot, unmanned aircraft controlled by a remote pilot, and semi-autonomous aircraft equipped for back-up control by a remote pilot.

In one embodiment, the inventive regional air transport network may include 4-classes of airports, most with runways >1,500 ft (or pads for VTOL aircraft), and differentiated based on their respective role in the regional network and the degree to which they are equipped to support high-frequency hybrid-electric flights:

-   -   Regional tier I, II and III airports. These are the primary         nodes of the regional network. Tier I airports are best equipped         for high-frequency electric flights, and offer fast recharge and         swap stations, and capabilities for all-weather and night         operations. Some of the tier I airports may also be served by         scheduled flights of conventional aircraft. Tier II airports         include fast recharge and swap station, while Tier III have         basic recharge capabilities on the tarmac. Unlike conventional         hubs, regional airports will offer fewer or a lesser degree of         ground services, e.g., baggage, security, given the relatively         lower traffic volumes and smaller aircraft. This will enable         quick transit through the airport, further reducing door-to-door         travel times;     -   Mainline large hubs in-region. A subset of the large commercial         hubs located in-region, with support for flights of small to         medium hybrid electric aircraft. These could include dedicated         short runways, non-interfering flight corridors, relatively fast         recharge and swap stations, quick passenger transfer from         regional electric to conventional air flights and vice versa.         Given that a significant fraction of the regional electric         flights will be “non-sterile”, the hubs could also include         provisions for this traffic to access sterile areas of the         airport, e.g., baggage and security services to “sterilize”         arriving regional passengers;     -   Regional service hubs. Airports in region equipped to service         and house the electric aircraft. These are typically a subset of         the regional tier I or II airports, and typically will include         parking, maintenance facilities, and operations centers; and     -   Cargo airports. Airports that enable regional transport of goods         between network hubs or distribution centers and local delivery         depots. These are equipped for high-frequency electric flights         just like the tier I, II and III airports described above, and         could include shared cargo and passenger facilities. These cargo         airports are typically located near points of origin of the         goods, e.g., network hubs, distribution centers, or points of         delivery of the goods, e.g., local delivery depots.

In some embodiments, the inventive hybrid-electric range optimized aircraft and associated regional air transport network may provide a relatively more quiet, cost-effective, energy efficient, and more convenient mode of transportation while also providing multiple related social and economic benefits. Such benefits include a reduction in the need to rely on automobiles for regional transportation, which would be expected to provide a reduction in pollution and traffic congestion. The inventive aircraft and system also may save passenger time, lead to an increase in productivity, encourage greater local development and housing, support decentralized living and working arrangements, and create new markets for connecting transportation services.

To permit realization of the opportunities presented by a more effective and efficient regional air transportation system, the inventors have recognized a need for several enabling devices, systems, data processing methods, and technologies. These include, but are not limited to a highly efficient and quiet short-take off capable hybrid-electric aircraft, and the associated and properly optimized technologies for regional operations “close-in” to communities and urban centers. In addition, there is a need for a regional transit network comprised of such aircraft, supporting airports, and the appropriate demand-supply matching mechanisms. Elements of embodiments of the invention are designed to address these and other needs. In particular, embodiments of the inventive system and methods may include one or more of:

-   -   Highly efficient plug-in series hybrid-electric powertrain         optimized for regional ranges. The powertrain may be designed to         minimize the energy required by sizing the powertrain for fast         cruise over a prescribed fraction of the range that represents a         majority of the flights, slower for longer ranges. This allows a         downsized generator, with power output less than required for         standard cruise, so that the energy storage units are used         continuously and fully depleted (less FAA required reserve)         during a flight. This also enables a relatively high energy         storage mass fraction in the range 12-20% of the total weight of         the aircraft. This higher ratio of electric storage to generated         power relative to conventional hybrid designs (and often with         generation optimized for cruise mode) is one key to the 65-80%         lower DOC (than conventional aircraft) delivered by the         inventive designs. Further reductions are enabled by         regenerative braking of the propulsors and all electric ground         operations;     -   Range optimized aircraft design enabling early impact of         electric air. Efforts to design commercial electric aircraft         to-date have focused on size, speed, and range capabilities         comparable to that of conventional aircraft. Given the range         times speed squared scaling of energy required for a flight,         this leads to designs that are either “mildly electric”, and         store only a small fraction of the energy onboard, or that are         more electric but require advanced electric technologies. This         has led to the view that electric air delivers limited savings         in the near-term, and that key technologies will take a decade         or more to mature. In contrast, by tailoring the inventive         aircraft to regional ranges, and lower speeds, altitudes, sizes,         the inventive “range optimized” designs can deliver         significantly lower DOC based on technologies that will be         available within significantly less time. This enables         market-entry many years earlier.     -   A built-in degree of “future proofing” (such as prevention of         relatively rapid technical or business related obsolescence) via         a modular, forward-compatible powertrain-propulsion coupled with         forward-compatible airframes. As with electric vehicle         technologies that are improving rapidly, a key barrier to early         adoption of electric aircraft is obsolescence driven by         technology evolution. This possible disincentive to adoption of         electric aircraft and the associated transportation system is         countered by a modular, forward-compatible design of the         powertrain, propulsion and airframe to enable technology         upgrades via simple module swaps. This enables early entry with         hybrid-electric aircraft that deliver continually improving DOC         via upgrades to stay abreast of energy storage technology and/or         improvements in operational efficiency. Another important         enabler is the inventive hybrid aircraft Powertrain Optimization         and Control System (referred to herein as “POCS”). This platform         adjusts operation of the modular powertrain based on         characteristics of the onboard energy storage units and         generator to deliver optimal performance. As a result,         technology upgrades are readily accommodated: flight objectives,         speed, efficiency, noise, payload are translated to control the         powertrain in a way that best leverages the modules onboard,         without need for extensive operator or pilot intervention;     -   Quiet operation with short-takeoff-and-landing (STOL)         capabilities to enable “close-in” flights and greater community         acceptance. Quiet STOL capabilities dramatically improve the         ability of an aircraft to fly “close in” to communities and         population centers, thereby delivering step-change reduction in         door-door travel times. STOL enables operations to smaller         community airports (>13,000 in the U.S.), bypassing congested         hubs. Quiet operation translates to greater community         acceptance, often a limiter for such flights. The inventive         system and aircraft leverage quiet electric ducted         variable-pitch fans (referred to herein as “eFans”) for         propulsion to reduce runway requirements and lower noise levels,         thereby enabling operations at a vast majority of existing         airports. The proposed inventive fan design has aerodynamics and         acoustics optimized for the intermediate speeds and altitudes of         the range-optimized aircraft. This includes use of a         low-pressure ratio variable pitch fan, enabling tailoring of         propeller blade pitch to flight mode for greater efficiency, and         use of regenerative braking to replace typically noisy spoilers.         The fan is powered by one or more high-density electric motors         located at the center of the duct and connected to the fan         directly, or through an optional elliptical reduction drive. The         high torque at low RPM of the electric motors coupled with the         high static thrust of the ducted fan leads to good STOL         performance. The combination of low fan tip speeds, fan-stator         and duct acoustic design and duct acoustic treatments deliver         significantly lower noise signatures. As an added benefit, the         increased safety and “jet like” appearance of the ducted fan are         expected to translate to strong consumer appeal relative to open         propeller aircraft often used for regional operation. The         aircraft and powertrain also include other features intended to         reduce cabin and environmental noise;     -   A distributed regional hybrid-electric air transit network for         passengers and cargo to enable effective large-scale operation         of the inventive electric aircraft. Aviation services today         require passengers (or transporters of cargo) to mold their         travel to the flight patterns of large, cost competitive         aircraft. In contrast, and as recognized by the inventors,         hybrid-electric technologies enable the opposite, to mold         aircraft and flight patterns to passenger travel needs. This is         implemented via a distributed regional electric air transport         network, operating out of a relatively large number of         neighborhood and community airports, and operating smaller         electric aircraft that are optimized to individual routes. The         form of this network will differ significantly from conventional         long-haul air transport networks and systems, leading to         distinct requirements for the constituent elements and processes         used to implement and operate the network. These are described         herein and include requirements for airports (including ground         transport options) and aircraft, to demand-supply matching         mechanisms. With regards to airports, in one embodiment this         includes 4-classes of airports all with runways >1,500 ft (or         VTOL pads) and differentiated based on role in the regional         network and degree to which they are equipped to enable         high-frequency electric flights. In terms of aircraft, in one         embodiment this includes hybrid-electric aircraft designed for         “lean” operations in-flight and on the ground at lower service         community airports. These elements are coordinated and their use         optimized using next-generation regional capacity management, to         improve aircraft load factors and utilization;     -   Development and use of a fault-tolerant design of the aircraft         powertrain for aviation-grade safety, a critical requirement for         large-scale application of hybrid-electric powertrain. In one         embodiment, this is addressed by designing the powertrain and         supporting optimization and control system (the “POCS” system)         for a relatively high-degree of redundancy to ensure continued         safe operation when faults occur. This may include features         offering redundancy in event of faults of the power sources,         converters, sensors or motors, among other elements or         processes. Other safety features may include those used to         prepare the powertrain ahead of a crash to ensure the platform         and modules respond to impact in ways to minimize risk to the         aircraft occupants;     -   Use of a powertrain designed for semi-automated optimization and         control, a factor that is critical for pilot acceptance and to         enable high-frequency operations at optimal efficiency. A key to         pilot acceptance of hybrid-electric aircraft is a control         platform with a simple pilot interface that mimics the operation         of a conventional aircraft. This platform (an example of which         is illustrated in FIG. 9, and described further herein) should         optimize the operation of powertrain modules to meet the         objectives of the pilot for the flight, across the integrated         powertrain (e.g., generator operation over course of a flight),         and for each module (e.g., motor RPM and torque for maximum         efficiency). In addition, the control platform (i.e., the POCS)         should support safe operation of the powertrain, through         appropriate fault isolation and recovery mechanisms. Other         features of the control platform may include streamlined         powertrain preparation and checks pre-flight, assisted         diagnostics and maintenance post-flight and simple calibration         following power module swaps. Many of these features or         requirements are enabled via the noted powertrain optimization         and control system (POCS) that serves as a single control         platform for the powertrain and its modules; and     -   Automated optimization methods for generating and correcting         flight paths for regional hybrid-electric flights. Note that         unlike long-haul flights by conventional jets, with well-defined         optimal altitudes and speeds, determining an optimal path for         one or more regional hybrid-electric flights (typically flying         at <30,000 ft altitude) is more complex. For example, the         differing operating characteristics of the various power sources         lead to varying optimal flight altitudes based on the degree to         which the generator is required during the flight. Thus, the         operating characteristics of the power sources need to be         considered, along with physical conditions during the flight         (e.g., terrain, weather and flight distance), and pilot         preference for the flight (e.g., high speed or economy) to         determine an optimal path. In some embodiments, this is enabled         via a Flight Path Optimization Platform (referred to herein as         “FPOP”, and described with reference to FIGS. 30 and 31) that         engages with the flight management system (FMS) and POCS to         define optimal flight paths and to refine these as conditions         evolve along the flight.

FIG. 18 is a diagram illustrating certain of the primary components, elements, and processes that may be present in an implementation of an embodiment of the inventive transportation system. As described herein, the inventive transportation system and associated apparatuses and processes may include a distributed air transit network for regional transport based on small to mid-sized (6-90 seats) hybrid and electric aircraft (having V/STOL capabilities). These are used to complement the current conventional long-haul air transport systems concentrated at a small number of hub airports.

The air transit network is tailored for high frequency operations of electric aircraft to a large number of regional airports currently not adequately served by conventional air, as well as low-impact operations into major hubs. This enables airlines, transit authorities, air-taxi, charter and cargo operators to offer profitable fixed or variable schedule and on-demand flights across the region at cost structures competitive with long-haul. The inventive transportation network offers significantly lower door-door travel times and lower total costs per mile than alternate regional travel modes: highways, rail or high-speed rail, conventional air. In some embodiments this is accomplished via convenient, high-frequency “close-in” flights to a large number of regional airports near communities and population centers, using the inventive quiet range-optimized hybrid-electric aircraft.

As shown in the figure, an embodiment of the inventive transportation network 100 may include one or more regional sub-networks 102. Each sub-network 102 may be affiliated with a region of a country, state, or other geographical region. Each sub-network 102 will typically include multiple cities and one or more regional or hub airports 104 from which operate one or more of the inventive aircraft 106. Each regional air or hub airport 104 may include elements and services to support the scheduling and “fueling” of aircraft, where here fueling refers to the recharge or swap of the stored energy units, and adding fuel for the range-extending generators (as suggested by “Recharge and refuel services” 108 in the figure). Management of the scheduling, refueling, and other services (such as record keeping) may be performed by one or more service platforms 110. Such platforms may include those used to access and process diagnostic information regarding flights, operate a fueling station, and schedule refueling operations. In some embodiments, service platforms 110 may include processes capable of performing supply-demand matching for scheduling flights, making parts available in an efficient manner, or other desirable matching or optimization processes related to management of the network and its constituent elements.

FIG. 19 is a diagram illustrating certain of the primary components, elements, data flows, and processes that may be present in an implementation of an embodiment of the inventive transportation system. As shown in the figure, such a system 200 may include an implementation of the inventive hybrid-electric regional aircraft 202. Aircraft 202 includes an embodiment of the hybrid powertrain 203 described herein, a powertrain optimization and control system (POCS) 204, a Flight Path Optimization Platform (FPOP) 205, a flight management system (FMS) 206, and a communications capability 207 for the transfer of messages and data to other components or processes of the system 200. A regional air transport operator 210 may include a set of processes for use in flight planning and other scheduling or administrative tasks related to the operation of one or more airports and their associated aircraft. Communications capability 207 may be used to transfer data related to aircraft payload, flight path, and energy state (among other parameters) to regional air transport operator 210. Data obtained from and/or processed by one or more of the aircraft 202 and transport operator 210 may be used to assist in flight scheduling via a regional capacity management platform or process 212, to assist in the administration and scheduling of the “refueling” processes via Recharge-refuel platform 214, or to assist in monitoring the operation of the aircraft during and post-flight (for purposes of pilot logs and diagnosing any issues) via a POCS online process or platform 216.

As suggested by the figure, demand for the regional air transport services may be driven by reservations of various types, and by the availability of aircraft, parts, and pilots. Such information 218 will typically be used by regional capacity management platform or process 212 to determine the appropriate number and type of fights made available to customers. Similarly, a fuel/energy/power services provider may use information related to flight scheduling, fuel needs, available fuel (such as charged modules), and sales/payments 220 to schedule refueling operations and accept payments for those operations via Recharge-refuel platform 214. An aircraft manufacturer 222 will typically provide information regarding the structure and operation of the aircraft and its systems to POCS online process or platform 216 for use in assisting a pilot or process to operate the aircraft and for diagnosing issues.

FIG. 20 is a diagram further illustrating certain of the primary components, elements, and processes that may be present in an implementation of an embodiment of the inventive transportation system 300. As suggested by the figure, the aircraft and pilot 318 may utilize one or more systems, platforms, modules, or processes (as suggested by “FMS”, “FPOP” “POCS”, “RRP onsite”) as part of scheduling or operating the aircraft. The Recharge-refuel platform onsite 314 (“RRP onsite”) assists the pilot with determining optimal recharge and refuel services required en route or at the destination by utilizing one or more systems, platforms, modules, or processes (as suggested by “Recharge and Refuel assistant”, “Service Provider database”, “Preferences”). Alternately, recharge and refuel decisions may be made by the Regional air transport operator 302 based on information provided to it by the aircraft and pilot 318. The Recharge-refuel platform onsite 314 similarly assists with these as shown. Information on recharge and refuel services requested by the pilot or the operator, and the service providers proposed schedule may be exchanged between the Recharge-refuel platform online 316 and the aircraft and pilot 318 or regional air transport operator 302 via a suitable interface 308. Recharge-refuel platform online 316 may utilize one or more systems, platforms, modules, or processes (as suggested by “service scheduling”, “service calendar and log”, “provider database”, “payment platform, “mapping platform”, etc.) as part of providing recharge and refuel scheduling, processing of payments for such services, etc. Similarly, data may be exchanged between the Recharge-refuel platform online 316 and the Airport fuel services provider 306.

As indicated, airports/airfields served by the inventive regional electric air transportation system may provide various levels of quick swap and recharge infrastructure to enable high-frequency electric flights. Recharge stations will operate to enable standard and fast charging of aircraft energy storage units in-situ, while swap stations will operate to exchange discharged or partially discharged energy storage units and replace them with charged ones. The inventive aircraft includes bays to house standard and extended energy storage units, and these may each be modular to enable removal of discrete modules comprising the standard or extended pack. As a result, the swap may involve replacing the existing modules with a smaller or larger number based on operator requirements such as the speed, range, payload and cost of the next flight.

Note that an aircraft's speed, range, payload and operational cost are determined to a large extent by the energy storage capacity onboard. As a result, the ability to add or remove energy supplying modules enables performance to be tailored to the needs of a specific flight. For instance, on a flight with less than design payload, the operator is able to reduce operating cost and/or increase electric range by adding energy storage units of weight up to the design payload minus actual payload, less the reduced fuel required. Conversely, the operator is able to accommodate payloads above design by removing energy storage units of weight greater than the payload overage plus the additional fuel required for the flight. This capability enables an operator to reduce costs on legs where the aircraft is loaded to less than capacity, and to accommodate overloaded flights. Further, in order to enable efficient module swaps and recharges, the transportation network may be supported by a software and communications platform 312 that enables pilots or regional air transport operators to determine energy needs and communicate these to fuel services providers at the destination airport, or at airport(s) on the way to the destination.

As noted, a block diagram of an embodiment of Recharge-refuel platform 304 is shown in FIG. 20. An aspect of its operation is illustrated by FIG. 20(A), which is a flowchart or flow diagram of an example process for determining recharge or refuel services required at a destination and in FIG. 20(B) for determining such services on the way to the destination. These processes or operations are executed by the “Recharge and Refuel assistant” module or process of the onsite aspects 314 of platform 304 based on pilot or operator request.

The processes or process flows illustrated in FIGS. 20(A) and 20(B) depend on multiple factors; these include payload and energy requirements of the route leg, onboard energy storage capacity and charge remaining, turnaround time and cost to determine swap and recharge requirements. These parameters and data are typically communicated to the Airport fuel services provider 306 together with flight details, ETA and turnaround time, so that provider 306 can schedule service and make preparations so that a swap or recharge is performed quickly and properly. To assist the pilot with recharge and refuel at a destination airport, the platform 304 determines the additional energy required for the next flight (such as a flight segment), and generates feasible options based on capabilities of the preferred service providers at the airport.

Such options may include one or more of tailoring the stored energy capacity to payload, adding stored energy units on low payload flights for improved energy efficiency, or removing units on flights where additional payload is required. Options may also include swap or recharge for the stored energy units based on one or more of cost, turnaround time, or impact on operating life of the stored energy units. The options are presented to the pilot along with the cost and time required, and the pilot's selection of a desired option is transmitted to provider 306 to schedule services. Similarly, to assist the pilot with services en route to a destination, platform 304 determines the range of the aircraft given the remaining energy onboard and the additional energy required for the next leg. This may be done in order to generate feasible pilot options based on service providers within range of the aircraft, along with the cost and time impact of each choice. Note that platform 304 may be used to support recharge and refuel planning for a single flight, for multiple flights in sequence, or for a flight with multiple legs. The sequence of services for a multiple-step trip is selected by the pilot based on guidance from the platform and transmitted to the service providers. During the course of the trip, recharge and refuel needs, and schedule are refreshed periodically based on progress of the flight, and transmitted to service providers whenever these change significantly or satisfy a specific rule or condition.

Recharge-Refuel Platform 304 also provides support for billing, payments and account management so that such transactions occur efficiently and using standard transaction authentication, authorization, and processing techniques. The energy storage units may be owned by the operator of the aircraft, in which case, swap units would be pre-positioned based on flight patterns, much as spare parts are today. The energy storage packs could also be owned by the services provider, or a 3rd party and loaned to the aircraft operator as a service. The services provider stores and recharges the spare packs, and swaps them as needed for discharged packs.

The Recharge-Refuel Platform 304 is comprised of a set of onsite functional modules 314 that are implemented onboard the aircraft or on-premises at the regional air transport operator, and a set of online functional modules 316 accessible via the internet or other suitable communications network. Note that although the services provided by an operator of such a platform will be referred to herein as recharge/refuel, they may also include an exchange of energy sources, and exchange may entail adding more, or reducing the total number of battery packs depending on operational needs. The Recharge-Refuel Platform connects and permits communications between hybrid-electric aircraft, regional air transport operators, and airport fuel services providers to enable highly efficient fueling operations. Elements of the platform may include one or more of the following:

-   -   Online service provider database online 320 and onsite 321 is a         periodically updated directory of airports, fuel service         providers at each airport, services capabilities of each         provider, services schedule, pricing and other logistical         details, e.g., affiliations, payment methods supported, etc.         Typically, the most current and comprehensive version of this         database is maintained within online platform 316. Abbreviated         (e.g., locally/regionally customized) versions of the database         are deployed as part of the onsite aspects 314 of platform 304         so that the onsite Recharge and refuel assistant module/process         322 can function without reliance on or connectivity to online         platform 316. However, note that as a backup, one or more of the         distributed sites may also maintain a copy of the comprehensive         version of the database; this redundancy may be of assistance in         providing recharge and refuel data to pilots and regional         facilities in the event of an interruption of services provided         by the central data repository, or to provide a pilot that is         significantly off course with assistance. The abbreviated         versions may be updated periodically from the online database,         when appropriately secure access is available and the updates         can be performed without having an undesirable impact on         operations;     -   Preferences data (elements, processes, or modules 324 and 325)         are a record of tailored settings for an aircraft or operator.         These may include default units, currencies and time zones,         preferred fuel service providers and custom pricing,         communication and transaction processes, as well as standard         fueling protocols for specific routes. These are stored onsite         324, as well as within the online platform 325;     -   Recharge and refuel assistant 322 enables the pilot or the         operator to determine optimal fueling required to support one or         multiple flights, and to select among available providers at an         airport or within range of an aircraft. The function or process         leverages the Provider database onsite 321 and the Preferences         data 324 of the Recharge-Refuel Platform 304, as well as a set         of modules or functions accessible on the aircraft or to an         operator, such as POCS and FPOP (the functions or operations of         which are described in greater detail herein);     -   Service scheduling module 326 receives specific fuel service         requests and attempts to schedule them with the requested         provider. If the requested time slot is available, then the         module returns with a confirmation and records the reservation         on the service calendar 328 for the aircraft. If the time is not         available, then the module returns with alternate openings.         Providers may give control of their schedules to the         Recharge-Refuel Platform, and/or manage schedules themselves.         Where the platform has control, module 326 schedules the service         on the provider's calendar and sends a notification to the         provider. Where the provider has control, module 326 notifies         the provider of the service request and waits for a         confirmation, or details on alternate openings;     -   Service calendar and log module 328 maintains a record of all         services scheduled by aircraft and by provider. For each past         service the module may track disposition, whether the service         was performed, an invoice for the service performed, details on         payments completed, outstanding feedback from the customer, etc.         Module 328 enables service providers to define service slots         available in the future, to permit the platform to book on their         behalf or retain control, to update their calendars to reflect         bookings made outside of the platform, etc.; and     -   Accounts module 330 is a record-keeping and transaction module         that enables providers to issue invoices and enables customers         to make payments. The module leverages standard payment         platforms 332 currently in use by pilots and operators, e.g.,         EDI, credit cards, EFT.

A further aspect of the inventive system 300 is the airport fuel services provider 306. This represents an operator or manager of an airport or airfield that is part of the inventive transportation system. Such an operator or manager may provide a set of services to enable aircraft to efficiently recharge or swap energy storage units, take on additional fuel for the range-extending generators, process payments for those services, etc. Provider of regional airport or airfield services 306 may interact and transfer data with Recharge-Refuel Platform 304 via a suitable interface 310.

Returning to FIG. 20(a), which is a flowchart or flow diagram of an example process for determining recharge or refuel services required at a destination, in one embodiment, the POCS (described in greater detail with reference to FIGS. 28 and 29) may be used to determine the available energy/fuel for the aircraft, the estimated tine if arrival, and the energy/fuel status after arrival (step or stage 350). Next, based on inputs from the pilot or flight scheduling processes, information or data concerning the next leg or segment of the flight may be received (step or stage 352). The FPOP process (described in greater detail with reference to FIG. 31) is used to determine the total energy required for the next leg or segment (step or stage 354). Next, the maximum available stored energy capacity for the next leg or segment is determined (step or stage 356).

Preference data (as described with reference to FIG. 20) may then be considered to determine the allocation of total energy required for the next leg or segment between stored (e.g., battery) and generated (e.g., based on the use of fuel). If such preferences exist (as suggested by the “Yes” branch of step or stage 358), then such preferences or conditions/constraints are used to determine the recharge and/or refuel requirements (stage or step 360). If such preferences do not exist (or are for some reason inapplicable, as suggested by the “No” branch of stage or step 358), then the recharge and/or refuel options may be determined based on availability, pricing, etc. (step or stage 362). As suggested by the figure, this determination may involve considering data contained in an airport service provider database. The determined recharge and/or refuel options may be presented to the pilot, and the pilot's decision(s) received (stage or step 364).

Based on the preferences and/or the pilot's decision(s), the recharge and/or refueling requirements are communicated to an appropriate service provider 367 (stage or step 366). This may include information regarding the flight, the aircraft, energy available and needed, the configuration of the energy sources, etc. After receipt and processing, the service provider 367 may provide a confirmation of the recharge and/or refuel order and any associated information to the pilot (stage or step 368).

Returning to FIG. 20(b), which is a flowchart or flow diagram of an example process for determining recharge or refuel services en route to a destination, in one embodiment, the POCS (described in greater detail with reference to FIGS. 28 and 29) may be used to determine the available energy/fuel for the aircraft, the estimated tine if arrival, and the energy/fuel status after arrival (step or stage 380). Next, the FPOP process (described in greater detail with reference to FIG. 31) is used to estimate the remaining range of the aircraft and determine the total energy required for the next leg or segment (step or stage 382). An airport service provider database may be used as a source of information and data regarding airfields having suitable recharge and/or refuel facilities (stage or step 384).

Preference data (as described with reference to FIG. 20) may then be considered to determine the allocation of total energy required for the next leg or segment between stored (e.g., battery) and generated (e.g., based on the use of fuel). If such preferences exist (as suggested by the “Yes” branch of step or stage 386), then such preferences or conditions/constraints are used to determine the recharge and/or refuel requirements (stage or step 388). If such preferences do not exist (or are for some reason inapplicable, as suggested by the “No” branch of stage or step 386), then the recharge and/or refuel options may be determined based on consideration of the impact of one or more recharge/refuel service options on the flight (as suggested by stage or step 390). This may involve considerations of turnaround time required and any expected delays to the flight, costs, airfield fees, etc. Based on the determined options and the application of any relevant rules, conditions, or constraints, a subset of the possible options may be determined and presented to the pilot (as suggested by stages or steps 392 and 394), and the pilot's decision(s) received.

Using the FPOP module or process, the aircraft's estimated time of arrival, stored energy, and available fuel may be determined (stage or step 396). Based on the preferences and/or the pilot's decision(s), the recharge and/or refueling requirements are communicated to an appropriate service provider 397 (stage or step 398). This may include information regarding the flight, the aircraft, energy available and needed, the configuration of the energy sources, etc. After receipt and processing, the service provider 397 may provide a confirmation of the recharge and/or refuel order and any associated information to the pilot (stage or step 399).

FIG. 21 is a diagram further illustrating certain of the primary components, elements, and processes that may be present in an implementation of an embodiment of the inventive transportation system 400. Referring to FIG. 21, in some embodiments, the inventive transportation system includes hybrid-electric regional aircraft 402, regional tier I or II airports 404, regional air transport operators 406, airport fuel services providers 408 and a Recharge-Refuel Platform 410.

As suggested by the figure, an embodiment of the inventive aircraft 402 may be equipped with a number of modular energy storage units: standard units 412 sized for use on flights at design payload, and extended units 413 for increased electric range on flights at less than design payload. These packs may be positioned for easy swap when on the ground using quick release mechanisms 414, in locations such as the wings, in pods suspended from the wings, under the fuselage. Aircraft 402 controls include a Powertrain Optimization and Control System (“POCS”, described in greater detail herein) 416, a Flight Management System (FMS) 417, and a secure datalink 418. The POCS 416 and FMS 417 may be implemented in form of a set of computer/software instructions executed by an electronic processing element, CPU, state machine, etc. Among other functions, POCS 416 tracks energy storage capacity onboard and energy remaining, FMS 417 estimates arrival times at the destination airport, and the datalink is used for communicating with the operator and fuel services providers.

The regional tier I or tier II airport 404 is equipped with a swap, refuel and recharge station 420 to enable quick turnaround of hybrid-electric flights. This includes equipment for automated or semi-automated removal and replacement of energy storage units, transport of the packs to and from storage, and a storage and recharge facility for energy storage units. Airport 404 may include a solar farm 422 for onsite electricity generation, and onsite grid storage 424 that is connected to the electric grid 426. Power to recharge energy storage units may be drawn in an optimal way across the solar farm, the grid storage and the grid, depending on requirements, cost, availability, etc.

Recharge-refuel platform 410 may connect entities across the air network to help orchestrate efficient recharges and swaps. The platform is engaged by pilots or air transport operators to identify/select providers and services based on operational needs. These requests are relayed to the providers who confirm and schedule service, and ensure stations are prepared for the arrival of the aircraft. Certain of the operations or functions that may be performed by platform 410 have been described herein with reference to FIGS. 19 and 20. Regional air transport operator 406 may operate to schedule and administer services for passengers, pilots, and aircraft. Certain of the operations or functions that may be performed by platform 406 have been described herein with reference to FIGS. 19 and 20. Airport fuel services provider 408 may operate to schedule and administer the provision of recharge and swap operations for the stored energy units (such as elements 412 and 413 in the figure) or adding fuel for the range-extending generators onboard the aircraft. Certain of the operations or functions that may be performed by platform 406 have been described herein with reference to FIGS. 19 and 20.

FIG. 22 is a diagram illustrating an example of the inventive range-optimized hybrid-electric aircraft 500 that may be used in an implementation of the inventive regional air transport system. In some embodiments, such aircraft and/or air transport system may have one or more of the following characteristics or qualities, where regional hybrid-electric aircraft are designed for optimal transport of passengers or cargo over regional ranges, typically up to between 500 and 1000 miles:

-   -   Aircraft are designed for “lean” operations in-flight and on the         ground, with one or more of the elements or processes described         herein, to enable air operations to small, limited service         airports;         This “lean” operation in-flight is enabled by one or more of the         following features of the aircraft:     -   Lower energy and cost: Aircraft and powertrain are optimized for         regional flights, e.g., lower speed, range and ceiling than         long-haul airliners. A powertrain optimization and control (POCS         or similar) platform is used to optimize energy use during         flights across the one or multiple energy/power sources;     -   Lower ATC load: Onboard ADS-B, including optional datalink to         air traffic control;     -   Fewer pilots: Fly-by-wire capabilities, including (if desired)         auto-land. Comprehensive FMS with operator upload. High levels         of automation including facilities for remote piloting or         fully-autonomous flight;     -   All weather operation: Pressurization for mid-altitude flights         (e.g., 25,000 ft) to enable weather and terrain avoidance; and     -   Minimal runway needs. Balanced field take-off <5,000 ft.         Soft-surface landing capabilities; In addition, “lean” ground         operations are enabled by the following features on the         aircraft, and at the airport:     -   Fast refuel and repair: Air-side quick recharge or swap         capability for onboard energy storage units (e.g., batteries).         Automated or manual transmission of refuel, recharge and         maintenance requirements via datalink ahead of landing;     -   Fast check-in and load: Cabins designed with storage racks near         doors to allow passengers to board with airline standard         carry-on baggage, making up for the low overhead space typical         of small to medium aircraft. Simultaneously, re-configurable         barriers may be used to separate passengers from a secure hold         for storing oversize and checked-in baggage. A simple plane-side         check-in platform (e.g. smartphone, tablet, PC) may enable quick         identity and ticket checks, and fee collection. Design will         support operation even in offline mode without network access,         via prior download of passenger and cargo manifests, and         execution of payments delayed till next covered by a network;     -   Flight prep: Comprehensive FMS with optional operator upload.         Automated system checks performed by POCS or other system.         Onboard aircraft monitoring platform with datalink; and     -   The regional air transport network may be supported by         next-generation capacity management capabilities to help         maximize aircraft load factors and utilization, such as:         -   Higher load factors: Regional reservations platforms,             including links to the GDS for conventional air. The             reservations platform operates to match customer demand in             real-time with available flights, including provisions for             fixed and demand-based scheduling (including near real-time             capabilities), on-demand and charter operations. Operators             engage with the platform via their private ARS (larger             operators) or via a variety of hosted, private labelled ARS             offerings (typically used by smaller operators); and higher             aircraft utilization: Virtual “pools” of electric aircraft             may be created, enabling owners and operators to offer and             rent aircraft for short (hours) or medium (days-weeks)             periods of time. Platform enables listing of pooled             aircraft, including availability and rental terms. Platform             includes streamlined processes for locating available             aircraft based on requirements, for negotiating terms and             contracting, payment processing and transfer payments, and             to receive or return aircraft. Similar virtual pools for             spare parts and pilots/crew enable quick turnaround,             schedule flexibility.

Returning to FIG. 22, the table(s) below provide a description of the primary elements of the aircraft illustrated in the figure, and also note the difference in construction, materials, and requirements between the inventive aircraft and conventional aircraft.

Required FIG. operational Element Description characteristics Difference from conventional 510 Standard energy storage Bays allow for rapid The modules may occupy the bays with access through energy storage same location as a traditional the lower wing skin. module swap (<5 chemical fuel tank in the main These bays are fully minutes for full wing. However, a tank is a utilized in normal aircraft). fully sealed, built-in unit operation which is neither accessed nor removable without wing disassembly. 511 Extended energy storage Same as 210 Same as 210 bays; utilization is optional allowing operators to trade payload against storage capacity, as is traditionally done with fuel 512 Energy storage pods: Pod is designed for Pod is similar to external fuel One or more energy quick swap tanks which have been used storage units enclosed by capability (<5 extensively by military fighter an aerodynamic fairing, minutes for full aircraft to extend range but mounted externally to the aircraft). Pod is self- very rarely used on aircraft, most likely in an contained, for commercial aircraft. under-wing location. cooling and safety requirements, e.g., BMS. 513 Energy storage bay Access same as 510. Conventional aircraft may located in the fuselage. utilize fuselage fuel tanks; Bay may be at multiple however, the use of such tanks locations fore and aft for variable CG location is along the fuselage to aid largely lost since fuel is burned in balancing the CG of off in flight. the aircraft. Bay may also be integrated with tracks to slide the energy storage unit fore and aft to modify CG of the aircraft. 519 Chemical fuel tank which Likely none may be located in the fuselage, the wing-body fairing, or may also be located in wing mounted fuel tanks 521 Aerodynamic fairing Minimize drag of the The requirement to cover (fair which encloses the range generator installation over) the air intakes and/or extending generator (527, while allowing fast exhaust for flight segments 526), and also access for without generator running is accommodates air intake maintenance, novel to the hybrid-electric requirements for Support modular system. Conventional engine combustion and cooling. powertrain capability nacelles have fixed inlets, by allowing different sometimes variable exhaust, fairings for different and engine is always running generators. Provide in flight. inlet and exhaust for cooling and combustion air. Inlet openings must be when generation is not in use to reduce cooling drag. 522 Electric propulsion High efficiency Conventional aircraft motors integrated within (>95%), high power propulsors designed to match the propulsor, in this case density (>5 kW/kg) engine while the electric motor a ducted fan. electric motor with may be integrated with any maximum suitable propulsor for the continuous power at aircraft design requirements. 2000-3000 rpm to match low noise propulsor. Motor capable of peak power rating up to 2x the continuous rating for limited time duration. 523 Quiet ducted fan - this is described in greater detail with reference to FIG. 23. 525 Electrical distribution system - this is described in greater detail with reference to FIGS. 24 and 25. 526 Generator and controller Generator operates at Conventional aircraft engines >95% and optimized include a starter-generator for the output RPM combination which starts the of the generation engine, and then absorbs a engine 527. Engine small fraction of the engine may be connected to power to run aircraft electrical the generator directly systems. or through a gearbox. The hybrid generator may or inverter-controller is may not function as the starter, solid state operating and utilizes 100% of the at better than 98% engine power to generate efficiency electricity for primary propulsion, and aircraft electrical systems. 527 Range extending High efficiency Large commercial aircraft may generators conversion of include a non-propulsion chemical potential engine, which provides energy to electrical auxiliary power and power. Engine pressurization air flow for controlled with a full ground operations, and authority digital following loss of a primary engine controller propulsion engine in flight. (FADEC). This generation engine provides energy for primary propulsion, as well as systems, and operates in combination with stored energy sources. 528 Ground power charge Provide a single No conventional equivalent. point: A single access connection point into point on the fuselage the main power which distributes grid distribution bus to energy to the energy allow simultaneous storage units for charging of all recharging energy storage units. May also include a connection to provide active cooling flow to the packs during high- rate charging operations. 533 Cockpit, forward High level of cockpit Even highly automated aircraft compatible for operation automation to allow require two pilots, and are not by two pilots (or a single for single pilot intended for remotely piloted pilot with ground backup operation without operation. assist), or unmanned for safety compromise. remotely piloted or Includes the autonomous operations. powertrain interface (POCS) and standard flight controls and navigation avionics. Additional provisions for a single pilot with ground assist, and flight controls which enable remote operation. 534 Powertrain Optimization and Control System (POCS) which serves as the pilot interface to the powertrain and the optimization processes (and which is more fully described with reference FIG. 26).

Note that with regards to the embodiment of the inventive aircraft shown in FIG. 22, this embodiment is a conceptual design of a range optimized, regional passenger aircraft. Electricity for the propulsion motors 522 is provided by a range-optimized series hybrid-electric powertrain (described further herein with reference to FIGS. 24 and 25), comprised of energy storage units 510 and range-extending generators 526-527 (left side only shown):

-   -   The energy storage units (in this case battery packs) are         located in the wings, including standard packs 510 and extended         packs 511 for use on flights with less than design payload. In         other embodiments, energy storage units may be positioned in         under-wing pods 512, and at various locations within the         fuselage. The embodiment shown has battery packs 513 positioned         under the passenger cabin in the forward fuselage. Fuel for the         range-extending generator is stored in a wing-body fairing tank         519;     -   Propulsion motors 522 in this embodiment are embedded in a         ducted fan 523 for high static thrust to enable short take-off         and landing, high climb rates, and quiet operations. Additional         noise reduction is achieved by locating the fans between the         V-tails 531 and above the fuselage, to shield the ground from         nose. The generators 527 are integrated within noise-insulated         aerodynamic nacelles 521;     -   Power to the propulsion motors is delivered by an electric         distribution system 525 which sources energy from any         combination of the stored energy units 510, 511, 512, 513, and         the range-extending generators 526, 527. Optimal sourcing of         energy from the storage units and generators is managed by the         powertrain optimization and control system 534 (the POCS,         described further herein with reference to FIGS. 26 and 27);     -   The aircraft is “plug-in” hybrid-electric, designed to recharge         stored electrical energy via ground-based charging stations via         a plug-in point 528, or by swapping fully or partially         discharged storage units for charged ones. Charging mechanisms         that connect to the mains or fast charge stations are included         onboard, enabling low or high-rate recharge in-situ. Storage         units are also equipped with quick release mechanisms to enable         fast swaps of storage units or modules thereof. Limited         recharging of storage units by the onboard generators is also         enabled, during low power operations in flight or on the ground;     -   All or most of the aircraft sub-systems may be electric, and         driven by hybrid-electric powertrain. These could include the         flight control system, landing gear, environmental control         systems, anti-icing, fuel pumps, taxi motors, and lighting; and     -   The aircraft may be equipped for a variety of flight modes,         ranging from conventionally piloted, to onboard pilot with         remote assistance, to remotely piloted, to fully autonomous with         remote assistance. As a result, the cockpit of the aircraft 533         may be configured for zero, one or two pilots, and may include         capabilities to enable control of the aircraft by a remote pilot         and by an auto-pilot unit.

An embodiment of the inventive range-optimized hybrid-electric regional transport aircraft 500 represents a relatively quieter, forward-compatible hybrid-electric aircraft optimized for regional passenger or cargo operations, either manned or unmanned. In some embodiments, such aircraft use a propulsion system powered by one or more electric motors, delivering thrust via propellers or other suitable mechanism, e.g., ducted fan (such as the inventive “eFan”, described in further detail with reference to FIG. 23). The aircraft is designed to operate with high efficiency in regional operations: distances <1,000 miles, cruise speeds and altitudes optimized for this range (<M 0.7, <30,000 ft), fuel burn typically 60-80% lower than equivalent conventional aircraft. The aircraft may be smaller (<100 seats) than conventional jets to match lower passenger volumes on regional routes, designed for shorter runway operations (<5,000 ft) to open up access to large numbers of smaller community airports, and operate with low cabin and environmental noise (<70 EPNdB sideline and cabin) for greater passenger and community acceptance.

FIG. 23 is a diagram illustrating a variable pitch electric ducted fan integrated propulsion system 600 that may be used in an embodiment of an electric-hybrid aircraft that is part of the inventive air transportation system:

-   -   The propulsion system 600 leverages inventive quiet electric         ducted fan propulsors (referred to herein as an “eFan”) to         enable critical quiet STOL capabilities. Quiet STOL dramatically         improves the ability of an aircraft to fly “close in” to         communities and population centers, thereby delivering a         step-change reduction in passenger or cargo door-door travel         times. STOL enables operations to smaller community airports         (>13,000 in the U.S.), thereby bypassing congested hubs and         saving passengers time. Quiet, efficient, reverse thrust may be         used for ground maneuvering in place of ground support         equipment, reducing the need for personnel and infrastructure,         which may be unavailable at smaller airports. Quiet operation         translates to greater community acceptance, often a limiter for         such flights;     -   As described herein, the inventors propose a novel         range-optimized design with aerodynamics and acoustics optimized         for the intermediate speeds and altitudes of the regional         hybrid-electric aircraft, emphasizing high efficiency in cruise         and high static thrust for STOL. This is enabled by use of a         low-pressure ratio (1.02 to 1.10) variable pitch fan, enabling         tailoring of propeller blade pitch to flight mode, including         reverse thrust, regenerative braking and feathering;     -   The eFan is powered by one or more high-power-density electric         motors located at the centre of the duct and connected to the         fan directly, or through an optional reduction drive. Liquid or         air cooling of the motors is fully integrated within the duct.         The transmission is fault tolerant, designed for continued safe         operation in event of motor, sensor or communication faults,         which either preserve, or allow for graceful degradation of         thrust output;     -   In addition to enabling the graceful degradation of thrust, the         variable pitch electric fan enables additional safety and         efficiency benefits not easily attained with conventional         propulsion systems and methods. The available high torque, high         response rate of thrust variation may be applied for         supplemental flight control, enhancing efficiency, and may         augment or replace flight controls completely (for example in         case of primary control failure);     -   In normal operation, control of the propulsor is via pilot or         auto-pilot commanding % power, % reverse power or % regenerative         braking, which are translated by the POCS system into         appropriate Propeller(s) blade pitch angles, Motor(s) RPM and         torque (or regenerative RPM and torque), and transmitted to the         motor and variable pitch controllers. In backup mode POCS         automation is bypassed, and the pilot commands the motor and         variable pitch controllers directly:         -   The POCS system translates % full-power to RPM, torque and             Propeller blade pitch angle based on the Power plan, %             power, Flight Mode, altitude and speed. For multiple             propulsors, commands may be synchronized, so all coupled             propulsors operate with the same settings;         -   Similarly, the POCS system translates % regenerative braking             or % reverse power to matching Propeller blade pitch angles             with motor set to appropriate levels of regenerative RPM and             torque; and         -   In case of emergency shutdown, the POCS system (or pilot             directly via the backup mode) commands the propeller blades             to feather position and halts motor motion.     -   The high torque at low RPM of the electric motors coupled with         the high static thrust of the ducted fan leads to good STOL         performance while the combination of low fan tip speeds,         fan-stator and duct acoustic design and duct acoustic treatments         deliver significantly lower noise signatures. As an added         benefit, the increased safety and “jet like” appearance of the         ducted fan are expected to translate to strong consumer appeal         relative to open propeller aircraft often used for regional         flights;     -   The propulsor is designed for forward-compatibility targeting         optimal efficiencies over a 30% higher speed band, with         structures designed to accommodate the higher torque and         gyroscopic loads of future motors;     -   Range-optimized design tailored for high cruise efficiency at         intermediate speeds and altitudes typical of regional operations         (Mach <0.7, Altitude <30,000 ft);         -   Forward-compatible design by selection of mass flow design             points over a cruise range that includes future maximum             speed and altitudes. This range extends from 30 to 250 mph             in equivalent airspeed, at Mach numbers <0.7. Fan cruise             pressure ratio is much lower, 1.02-1.10 relative to             high-speed jet engines for high net installed efficiency,             especially in climbs and lower altitude, lower speed cruise             operations. Inlet and exhaust areas are selected to avoid             separation and distortion over this range of mass flow             conditions; and         -   Variable pitch fan disk and blades 601 to enable high             efficiency over the targeted speed range.

In some embodiments, the eFan design consists of:

-   -   A fan disk with a plurality of fan blades (6-20 blades), and a         disk solidity in excess of 60%;     -   Fan blades designed for high efficiency at low pressure ratios         and operation at 3000-4000 RPM. This entails an increasing         aerodynamic loading with span, and corresponding increase in         chord;     -   Fan tips may be a spherical cross section to allow variation of         pitch within the matching duct wall contour while maintaining         the small tip clearance required for high efficiency; and     -   Fan blades designed for optimal efficiency over the targeted         cruise speeds, extending to future maximum speeds and altitudes.         Including design for static thrust, reverse thrust and         regenerative braking via the variable pitch capability;     -   Fan blades are mechanically pitched over a wide range of angles.         Fan pitch angle is measured such that 0° aligns the blade tip         chord plane with the plane of rotation;     -   Fan blades are variable pitch with angle change at speeds         >100°/sec;     -   At minimum, the variable pitch mechanism will accommodate the         normal operating range between 15° for fine pitch on takeoff, up         to 50° in high speed, low RPM cruise;     -   Maximum positive angle may be up to 80° for the “feathered”         position of minimum drag, as blades are aligned with incoming         flow; and     -   Minimum angle may be up to −40° to enable reverse thrust while         maintaining continuous motor and fan rotation.

As shown in FIG. 23, fan blades 601 are attached at the root 611 to a mechanical hub with a mechanism 610 for electro-mechanical variation of blade angle (pitch) from a negative angle providing reverse thrust for enhanced runway braking, to a fully streamlined angle for minimum drag in case of propulsor shut-down in flight. The entire mechanism rotates with the fan disk and electric drive motor. Blade pitch change signal is passed across the rotating boundary. A mechanism drives all of the blades simultaneously through a mechanical linkage. The design includes a no-back, directional brake to lock out feedback torque from the mechanism during periods of no pitch change.

The eFan may be installed in an aerodynamically contoured flow duct 603 to deliver the noise reduction and static thrust required for quiet STOL operations. In one embodiment, the duct axial length is 50-125% of diameter, with the fan located at 40-60% of duct length. The duct is supported by a plurality of stators 602 located behind the fan disk. The duct inlet lip contour 604 is of continuously variable radius designed for high efficiency in cruise, no separation at low speeds and high power, and reduced propagation of forward fan tones. The duct inlet lip contour 604 ahead of the fan promotes laminar flow while minimizing separation. The duct contour aft of fan is sufficiently gradual to avoid flow separation in the normal operating envelope. The duct exit area minimizes jet noise by expanding flow aft of fan, reducing flow to near free stream levels. The duct outer contour 603 is designed to maximize natural laminar flow for low drag. The duct internal cross-section may include a radial recess or other mechanism aligned with the fan to enable the small tip clearance required for high efficiency.

The inventive eFan 600 may be characterized by one or more of the following:

-   -   It is designed for low-noise operation, with 15-25 EPNdB lower         noise than conventional aircraft, as enabled by one or more of         the following features:         -   Shorter blades in ducted fan relative to an equivalent             thrust open propeller translate to quieter operation due to             reduced tip speeds, target 500-600 fps, upper limit of 800             fps, and attenuation of radial noise components by the duct             and duct insulation. In addition, blades are optimized for             low noise including leading edge sweep angle, trailing edge             shape, blade tip and root shapes, and blade tip to duct             clearance shape with varying pitch;         -   Rotor-stator noise reduction via stator design and placement             for low noise         -   The number of stators is optimized for noise and determined             by number of fan blades and blade rpm to ensure primary, and             secondary BPF fall below 2500 Hz. (BPF=blade passage             frequency);         -   The stator spacing behind blades is optimized for noise             reduction, 1.5 to 2.5 blade chords aft of the fan. The             stator twist and platform is designed to remove flow swirl             to reduce turbulent eddy noise;         -   Use of variable pitch blades reduces wake intensity, the             main driver in rotor-stator noise, especially at takeoff;         -   The duct is designed to attenuate noise, including optimized             axial location of the fan in duct, design of duct lateral             profile, inlet lip contour and exit profile to minimize             propagation of fan tones, acoustic treatments of critical             areas of the duct inlet, central fairing, and outlet; and         -   The duct may be used as a variable drag air brake, replacing             conventional spoilers which are a significant source of             airframe noise;     -   It is designed for energy recovery and aircraft speed control         via regenerative braking to improve overall efficiency, and to         eliminate the need for a typically noisy air brake mechanism.         Regeneration, and hence airspeed control, is fully variable, and         is enabled by adjustment of variable pitch propellers and the         electrical load applied to the motors. A pilot may request %         regenerative braking using the standard power lever angle,         moving into a guarded range below standard flight idle. The POCS         system delivers % regenerative braking by controlling propeller         blade pitch angles and motor regenerative power output to         deliver target levels of aerodynamic drag measured via motor         power output;     -   It is designed for reverse thrust for reduced stopping         distances, especially on surfaces with reduced braking action,         and also for ground operations requiring reverse (e.g., standard         gate “push-back”), reducing need for airport operations         infrastructure. Reverse thrust may be enabled though a variable         pitch fan with the blades pitched to a negative angle, or may be         enabled by reversing the motor direction of rotation. Reversed         rotation is a capability unique to the electric fan, not         available with a conventional aircraft engine without         complicated gearing;     -   It is designed for aircraft supplemental or primary control. The         high constant torque, millisecond fast motor response, and high         speed fan pitch rate response enable the ducted fan to quickly         change thrust output. This differential or vectored thrust         produces moments around the aircraft center of gravity which may         be utilized to provide primary or supplemental aircraft control.         In case of primary control failure, control system may be         reconfigured to utilize thrust moments to restore some degree of         lost control authority:         -   Differential thrust. In one implementation, the thrust from             one or more propulsors may be varied to provide a moment             around the center of gravity. Depending on motor location,             and number of propulsors, this may produce a pitch or yawing             moment;         -   Vectored thrust. In a more active implementation, thrust             from one or more propulsors may be vectored through use of             exhaust louvers, propulsor gimbals, or other means to             produce a pitch, yaw, or rolling moment;     -   Ducted fan may be designed for lift augmentation either         directly, in which thrust from one or more propulsors is         vectored through louvers, gimbaled mounting or other means in         order to generate a thrust vector which directly offsets         aircraft weight (i.e. lift), or indirectly, by channeling         exhaust flow over aerodynamic surfaces to produce suction         (lift), and/or flow deflection (e.g., Coanda surfaces such as a         “blown flap”);     -   It is designed for integrated cooling. Electric motors and the         related controller-inverter electronics produce a significant         amount of waste heat. It is highly desirable that heat rejection         be accomplished with minimal added weight and drag. This may be         implemented directly into the ducted fan design in the following         manner;         -   Heat exchanger surfaces may be incorporated into the             stators, and/or the inner, aft surface of the duct. In this             way, there is no additional radiator and no additional             surface area for drag, especially important since heat flow             varies directly with power output, and may drop to zero in             flight during descent, at which time cooling penalty is             desired to be negligible;         -   Motor heat may be rejected to a heat exchanger in the             leading edge of the nacelle to prevent ice buildup when             flying in freezing precipitation; substantially more energy             efficient than providing power to an electro-thermal hot             leading edge;     -   Note that the eFan design is a fault-tolerant architecture, as         exemplified by the following features:         -   The assembly is designed to ensure continued safe operation             with graceful degradation of thrust in the event of a fault             in any one motor system (including motor inverter,             controller, power bus etc.), as enabled by POCS (element             1042 of FIG. 27 and/or element 1160 of FIG. 28). Hardware             designed to support graceful thrust degradation, including             multiple electric motors may power a single shaft,             electrical isolation ensures that a fault in one does not             affect safe operation of the other(s), and individual motors             may be designed for peak performance 60-80% above continuous             for recovery periods of 5-10 minutes, so that surviving             motors are able to power up to partially or fully             accommodate faults elsewhere. This may include designing the             motor for higher power ratings and introducing mechanisms             for aggressive cooling of hot spots, so that extended             peaking does not damage the motor. In the event of a loss of             motor power due to a motor fault, the POCS alerts the pilot             and redistributes power to the healthy units to preserve             thrust for sufficient time durations to allow the pilot to             manoeuvre to safety (element 1144 of FIG. 28);         -   In case of the total failure of a propulsor, including             failure due to physical damage, blades may automatically be             set to “pinwheel” (blades continue to rotate, but no energy             extracted from the flow for minimum drag), or pitched to a             fully streamlined angle (“feathered”) and the motor then             braked to prevent any rotation. Failure or potential failure             may be detected through monitoring of the motor power output             vs. commanded output, and by monitoring vibration at the             motor to detect mechanical damage/failure; and         -   Fault tolerance in event of communications or sensor             failures may be achieved via redundant systems. Standard             connection of the motor and variable pitch controllers to             POCS is supplemented by back-up wiring, including capability             for direct access to the controllers without POCS             intervention. Similarly, motor and pitch sensors are             supplemented by back-up sensors or a sensor-less control             capability. The sensor fault detection capability within             POCS switches across these as required.

Returning to FIG. 23, the table below provides a description of the primary elements of the eFan illustrated in the figure, and also note the difference in construction, materials, and requirements between the inventive eFan and conventional fans/propulsers.

Required operational Difference from FIG. Description characteristics conventional 601 Low pressure, variable Fan blade pitch range Conventional turbofan pitch fan with 6-20 blades, from 15 to 50 degrees blades are fixed pitch, high a disk solidity of >60%, (normal operations), up pressure ratio (1.4 to 1.8) and pressure ratio of 1.02 to 80 degrees (feather), and disk solidity >1 to 1.10 and −40 degrees (reverse thrust), cruise efficiency >95%, cruise pressure ratio <1.10 602 Stators aft of the fan Low noise operation; Fan: stator noise remove the swirl from the fundamental blade interaction is significantly flow, reducing turbulence passage frequency different (and less losses, and support the <2500 Hz, External noise dominant) with duct allowing a very small <70 EPNdB at 500 ft conventional high fan tip clearance. Stators sideline takeoff pressure fan may also function as a measurement. radiator with embedded cooling fluid coils. 603 Ducted fan nacelle which Outer surface designed Conventional jet nacelles minimizes external and for minimum provide a substantially internal drag. Nacelle aerodynamic drag, with different flow contours, length optimized to meet up to 50% laminar flow, both internal and external requirements in at Mach numbers <0.7 performance, noise Internal contour to attenuation, weight, and minimize drag, attenuate drag, and may range from fan tonal noise, and 50% to 125% of diameter maximize thrust generation efficiency. Internal contour may preserve free stream velocity or allow flow acceleration. 604 Nacelle leading edge Leading edge radius Conventional turbofan sufficient to prevent nacelles compromise static separation at high power, thrust for cruise at high low speed operations Mach number. Ice (high static thrust) while protection uses hot bleed providing low drag in air from the high pressure cruise, including pressure turbine. gradients favorable to natural laminar flow. Leading edge thermally heated for ice protection. 610 One or more electric Energy density >5 n/a motors kW/kg continuous rating, with peak power at <4000 rpm 611 Variable pitch hub Fast (>100 deg/sec) Conventional turbofans are simultaneous adjustment fixed pitch. of blade pitch angle from extreme negative to extreme positive.

FIG. 24 is a diagram illustrating a powertrain 700 and its associated elements that may be used in an embodiment of an electric-hybrid aircraft used as part of the inventive air transportation system. As shown in the figure, in one embodiment the powertrain 700 and associated elements may include or be characterized by one or more of the following features, elements, processes, or aspects:

-   -   A series hybrid-electric powertrain delivering power via one or         more electric motors, combining batteries (or other method(s)         for storing electrical energy) with a chemical fuel based engine         and generator combination as an optional range extender. The         engine could be piston, turbine or other form of heat engine to         convert stored chemical energy to electricity. The powertrain         also delivers power to other electric sub-systems of the         aircraft which could include the flight control system with         electric actuators, electrically actuated landing gear,         environmental control systems, taxi motors, anti-ice, fuel         pumps, and lighting;     -   The powertrain comprises a set of modules, e.g., battery packs,         engine, generator, power inverters DC/DC converters, fuel         system, electric motors, etc. integrated via a powertrain         platform comprising power and control circuits. Each module is         connected via a control circuit to the powertrain optimization         and control system (POCS). Module controllers are queried or         directed by the POCS platform, and transmit a range of state and         performance information to POCS on-demand or continuously. POCS         to module controller communication is enabled by APIs, defining         protocols by which POCS and modules communicate;     -   The operation of the powertrain is controlled by POCS based on         pilot direction, in semi-automated or fully-automated mode(s).         To enable this, individual powertrain modules are equipped with         controllers that communicate with POCS across the module         interface via APIs, on-demand and/or periodically. Key metrics         communicated to POCS may include the following: on-off, RPM,         power, status for each motor; battery capacity, power, status         for each battery pack; fuel level and flow rate; engine on-off,         power, status; and status for each converter. Key control         directives received from POCS include the following: on-off, RPM         and torque for each motor; power for each battery pack; on-off         and power for the engine; and     -   The powertrain is “plug-in” and designed to recharge stored         electrical energy via ground-based charging stations. Limited         recharging in flight may also be enabled: by the engine during         low power operations; and/or by means of regenerative braking of         wind-milling propulsors during descent and of landing gear         following touchdown. As noted, energy storage units may be         contained in multiple modules, installed internal or external to         the aircraft, for example in the wings, with an optional quick         release mechanism to enable fast swap or jettisoning. Included         onboard are charging and cooling mechanisms that connect to the         mains or fast charge station for low or high-rate recharge         in-situ.

Referring to FIG. 24, the powertrain 700 includes one or more electric propulsors 701, one or multiple distribution buses 730, one or more rechargeable energy storage units 710 and if desired, one or more optional range-extending generators 720. The powertrain 700 may also include element 731 to power the distribution buses 730 from an external source, element 713 to charge the rechargeable storage units 710 from an external source, and element 732 to distribute power to other electric systems of the aircraft. Note that elements 713, 731, and 732 may take any suitable form, such as (but not limited to) an electrical interface, a cable, a coupling, or a controller. Whatever its form, element 732 typically includes one or more DC-to-DC converters to convert power to the lower voltage levels typically required by the other electric systems, e.g., environmental control systems, fuel pumps, anti-icing, lighting, as well as back-up/fail-safe distribution for vital systems, e.g., flight controls and avionics.

Powertrain 700 is a plug-in series hybrid designed to power the electric propulsors 701 with energy drawn optimally from the rechargeable energy storage 710 and the range-extending generators 720. Given the typically lower total cost of energy from rechargeable energy storage 710, power is drawn from the range-extending generators 720 only if stored energy is insufficient to complete the flight, or if maneuvering requires power beyond that available from the rechargeable storage 710. The total cost of energy from the rechargeable energy storage units equals the cost of the energy used to charge the units, the efficiency of charge and discharge of the units, and the cost of the units amortized over their usable life, defined as number of charge-discharge cycles before performance degrades below a threshold. Cost effective battery packs, for instance, can be charged using low-cost electricity from the grid, and offer very high efficiency charges and discharges, with a usable life of >1,000 cycles.

Electric propulsors 701 are either ducted fans as shown (such as those described with reference to FIG. 23), or open propellers. The propulsors are designed for operation in multiple modes through the variable pitch mechanism 703 shown, or by other means such as an adjustable exhaust plug. The operational modes enabled could include take-off, cruise, regenerative braking, feathering, reverse thrust, for example. Fan 702 is mechanically coupled to one or more electric motors 704, with a mechanism or process to isolate individual motors to enable continued operation in the event of mechanical or electrical faults. In normal operation, fan 703 is driven by electric motors 704, which receive electric energy from the distribution bus 730 via the motor controller and DC-AC inverter-rectifier 705. In regenerative breaking, on the other hand, fan 703 drives the electric motors 704 to generate electric energy that is delivered to the distribution bus 730 via the DC-AC inverter-rectifier 705.

Rechargeable energy storage units 710 are comprised of battery packs 711 shown, supercapacitors, or other media for storing electrical energy (or a combination thereof), coupled with a battery management system(s) 712 that manages operation and safety of the packs. Each pack may comprise of multiple individually removable battery modules, and operate either with some or all of these modules in place. Storage units 711 are charged primarily by external sources via 713, but also enable limited charging in flight, by the electric propulsors 701 during regenerative braking, or by the range-extending generators 720 during low-power flights. The rechargeable storage units 710 deliver power to the distribution buses 730 when discharging, or receive power from the distribution buses 730 or external source 713 when recharging.

Storage units 711 are equipped for fast charge in-situ via external sources 713, and also equipped for fast swap with quick release mechanisms. These enable a manual or automated swap of on-board storage units with pre-charged replacements positioned on the ground.

The optional range-extending generators 720 may be comprised of internal combustion engines 721, each driving one or more generators 723. Alternately, these could be comprised of units that convert stored chemical energy directly to electricity, e.g., hydrogen fuel cells. The internal combustion engine 721 may be a conventional one, using one of a range of fuels, e.g., diesel, gasoline, jet-A, for initiating and sustaining combustion in one or more combustion chambers. The fuel is stored in one or more fuel tanks 722, and pumped to the generators as needed. The engine 721 is mechanically connected to the generators 723, typically with a mechanism or process for isolating individual generators in the event of a fault. When operating, engine 721 drives the generators 723 to deliver electric energy to the distribution bus 730 via AC-DC rectifiers or inverters acting as active rectifiers 724.

FIG. 25 is a schematic of a series hybrid drive configuration 800 for a representative aircraft that may be used in implementing an embodiment of the inventive transportation system. Note the following features, elements, processes, or aspects:

-   -   The powertrain includes two electric propulsors 801, each         powered by two electric motors 802, two battery packs as         rechargeable storage units 803, and a single range-extending         generator. In this example, the generator couples a single         internal combustion engine 804 coupled with two motor generators         805;     -   In some embodiments the electric motors 802 are >90% efficiency         brushless, electronically controlled axial-flux drive motors,         with specific power density >5 kW/kg at continuous output power,         and peak power output >50% above continuous output. In addition,         the motors may be designed for low RPMs, e.g., <4,000 to enable         direct drive. The motor generators 805 are of the same         architecture as the drive motors, able to operate at peak for a         recovery period, e.g., failure of a battery pack. Each motor 802         and generator 805 is coupled to a solid state         convertor-controller (such as a rectifier), to provide precise         motor control with minimal loss and to protect the motors from         voltage fluctuations;     -   In one embodiment, the internal combustion engine 804 is a         turbo-diesel piston engine, tuned to operate with maximum         efficiency at a fixed RPM that may align with the design RPM of         the electric motors to enable direct drive. The turbo-charging         allows the engine to deliver relatively uniform power from sea         level up to 10,000 ft;     -   Power is delivered to each of the propulsors 801 by one of two         primary buses 806, each of which is powered by one of the two         battery packs 803, and one of the two motor generators 805. The         primary buses 806 also distribute power to the non-propulsion         sub-systems of the aircraft 810, via step-down DC-DC convertors         807;     -   A third vital bus 808 re-routes power to accommodate failures of         any of the electric motors, distribution buses, battery packs or         generator. In the event of failure of an electric motor 802, the         vital bus 808 re-routes power to the surviving motors, enabling         the pilot to request peak thrust for recovery maneuvers. In         event of failure of a primary bus 806, the vital bus 808 engages         to fully replace the lost functionality. In event of failure of         a battery pack 803 or generator 805, the vital bus 808 re-routes         power from surviving sources to maintain balanced output from         the electric motors; and     -   The vital bus 808 also re-routes power to the non-propulsion         sub-systems 810 and avionics 812 in the event of failure of a         primary bus 806 or one of the step-down DC-DC convertors 807.     -   810 and 812 shown are representative of standard circuits used         to power non-propulsion sub-systems and avionics onboard an         aircraft. The former includes systems such as Ice protection,         the Fuel pump, Pressurization, Cooling, Flight controls, and         operates at an intermediate voltage, e.g., 270V. The latter         operates at a low voltage, e.g., 28V, and includes the most         critical avionics systems on an aircraft. As shown, these         circuits typically include redundant paths and additional power         sources for fault tolerance in the event of failures.

As described with reference to FIG. 22, the inventive transportation system includes an aircraft design optimized for maximum transport efficiency over regional ranges, in particular, the innovative range-optimized hybrid-electric powertrain. In some embodiments, this design goal contributes to the following features, which collectively enable a 65-80% lower DOC over targeted regional ranges than conventional aircraft:

-   -   A powertrain sized for maximum transport efficiency over         regional ranges <1,000 miles, designed via a 3-level or 3-tier         objective:         -   (A) Highest efficiency (80+% lower DOC than conventional             aircraft) and optimal speed over electric-only range;         -   (B) Intermediate efficiency (60-70% lower DOC than             conventional aircraft) and optimal speed over larger hybrid             range; and         -   (C) Good efficiency (30-60% lower DOC than conventional             aircraft) and lower speeds beyond to maximum range             determined by onboard stored energy and fuel less safety             reserves;     -   A powertrain sized for speeds and altitudes that are optimal for         regional sub-range (B), determined by minimizing an objective         function, for example “DOC+I+COT” for flights over the sub-range         (may also be optimized for lower speeds over the regional         sub-range (C) based on the relative frequency of travel over         range (C) versus ranges (A) and (B)). This leads to design for         slower speeds, lower altitudes and shorter ranges than that of         conventional jet aircraft;     -   A rechargeable energy storage and range-extending generator         combination, sized based on speed and range requirements         (A), (B) and (C). Stored energy-first design, whereby         rechargeable energy storage is fully depleted for flights in         ranges (B) and (C) with required reserve maintained as fuel for         the optional range-extending generators, or less required         reserve if no range-extending generators are on board.         Rechargeable energy storage and range-extending generators sized         to enable optimal speeds over hybrid range (B), and rechargeable         energy storage sized to enable optimal speeds over electric-only         range (A). Range-extending generators sized to enable         lower-speed cruise over range (C), and therefore downsized to         under 70% of maximum continuous powertrain output (much lower         than conventional aircraft) for improved efficiency; and     -   Optimized energy storage mass, 12-20% of the aircraft weight,         and downsized range-extending generator, with very low power         output, typically less than 70% of maximum continuous output of         powertrain (lower than a conventional aircraft).

Note that a design process for an embodiment of the inventive range-optimized aircraft and powertrain is described herein, including a process by which the noted 3-tier set of speed and range requirements is used to size elements of the hybrid-electric powertrain. The described designs for the inventive aircraft and associated elements are forward compatible to support anticipated upgrades of operating capabilities or key powertrain modules over the life of the airframe. Given the rapid evolution of EV technologies, this feature ensures that the powertrain remains competitive over time as individual module technologies improve, e.g., batteries, supercapacitors, electric motors, internal combustion engines, fuel cells. In addition, this feature enables the aircraft to transition smoothly from hybrid-electric to all-electric once energy storage technologies improve to the point where range-extending generators are no longer required.

To provide forward compatibility, the powertrain is designed by sizing the energy storage units and generator combination for the speed and range requirements (A), (B) and (C) mentioned, based on technologies available at aircraft launch and forecast to be available over the 15 year period beyond (including planning for transition from hybrid-electric to all-electric). This leads to a forecast for onboard rechargeable storage and range-extending generators, and in turn, determines performance characteristics over time: speeds, electric and hybrid-ranges and operating costs; with electric-ranges increasing and operating costs decreasing as technologies improve.

Forward compatibility may require limiting the weight of the rechargeable energy storage units to 12-20% of the aircraft weight so that payload capacity is roughly uniform as EV technologies improve. Higher weight fractions would lead to designs that are larger and heavier than aircraft of similar payload in the initial years, with payload increasing over time. Lower fractions lead to suboptimal efficiencies given much higher use of range-extending generators.

To achieve forward compatibility, the powertrain platform is designed to support module technologies over the design life of the airframe (typically 15-20 years). This can be realized by designing the platform based on powertrain operation with future modules where appropriate, and ensuring upgrades required to accommodate future technologies are relatively simple and cost effective. For instance, wiring to the electric motors may be rated up to 30% higher peak power, to support more powerful motors and higher aircraft speeds in the future. Wiring of the platform may be designed to allow upsizing and redistribution of the rechargeable storage units, downsizing or removal of the range-extending generators. Wiring from the energy storage units may be designed to support higher capacity packs in the future, and space used for the range-extending generators may be wired for use with rechargeable storage units when the generators are removed. In addition, modules and elements of the powertrain e.g., wiring, harnesses, switches, converters, that will likely require upgrade are designed and positioned for simple replacement and easy access.

The powertrain platform and the powertrain optimization and control system (POCS, described with reference to FIGS. 26-28 and the example powertrain configurations illustrated in FIG. 24-25) are designed to enable a staged transition of the powertrain from hybrid to all-electric as storage technologies improve. This includes design for operation with or without the optional range-extending generators, fuel or rechargeable storage based reserves, and a platform designed to allow the swap of generators with storage units over time. In addition, the powertrain may be characterized by the following:

-   -   Modularity—a set of swappable nearly “plug and play” capable         modules connected by a hardware and software platform. This         enables the powertrain to accommodate rapidly improving         technologies via relatively simple upgrades of the modules.         Powertrain modules may include the rechargeable storage units,         range-extending generators and electric motors. The powertrain         platform includes the powertrain optimization and control system         (POCS), electrical wiring, distribution buses, convertors, fuel         system, sensors, cooling, shielding, and any additional         processes or structure that operate to enable the modules to         cooperate to form the powertrain;     -   Modularity is facilitated by design of the powertrain platform         and the interfaces that connect the modules to the platform to         be compatible with the range of module technologies that are         likely to be available over the life of the airframe (as         mentioned previously). This enables compatible modules to be         plugged into the powertrain by connecting the module to the         interfaces, comprised of electrical and control circuits, and         services such as cooling, shielding, fuel, and structure. For         instance, range-extending generators plug-in to the platform via         electrical connectors to the generator rectifiers, via POCS         connectors to the generator controllers, the internal combustion         engine controller and the fuel system controller, and via fuel         and cooling services to the generators and engine. In areas         where upgrades may be required to accommodate a new module, the         powertrain is designed to enable relatively simple and         cost-effective modification;     -   Individual module controllers are connected to the powertrain         optimization and control system (POCS) to orchestrate operation         of the powertrain. Module controllers are queried or directed by         the POCS platform, and transmit a range of state and performance         information/data to POCS on-demand or continuously. POCS to         module controller communication is enabled by APIs, defining         protocols by which POCS and modules communicate. The operation         of the powertrain is controlled by the POCS based on pilot         direction, in semi-automated or fully-automated modes. To enable         this, individual powertrain modules are equipped with         controllers that communicate with POCS across the module         interface via APIs, on-demand and periodically. Key metrics         communicated to POCS may include the following: on-off, RPM,         power/status for each motor; battery capacity, power/status for         each battery pack; fuel level and flow rate; and generator         on-off, power/status. Key control directives received from POCS         may include the following: on-off, RPM and torque for each         motor; power for each battery pack; and on-off and power for the         generator;     -   The powertrain is designed to support relatively simple module         exchanges/swaps. The powertrain platform and interfaces to the         platform, electrical, control, and services such as cooling,         shielding, fuel, structure, are designed to accommodate a wide         variety of modules. These include specification of corresponding         wiring, control or monitoring and other service capabilities         onboard each module to enable a type of “plug and play” pairing.         For instance, in the case of battery packs, this would typically         include peak and steady-state discharge rates, BMS protocols,         socket descriptions. POCS also enables calibration of the         powertrain following module changes. This may include FAA         certification of the powertrain for use with a range of         pre-approved compatible modules. Further, in some cases, the         design of the powertrain may include the ability to support         relatively simple or cost-effective modification in areas where         new modules may require modification;     -   Powertrain variants may be implemented that have performance         tailored to different markets; in some cases this may be done by         altering the choice of powertrain modules, to offer different         speeds, ranges and operating costs for an aircraft         configuration. For example, an “economy” commuter powertrain         might couple a highly efficient turbo-diesel range extender with         moderate density batteries, offering best-in-class operating         costs but long flight times for longer ranges. In contrast, a         “performance” business powertrain might couple a less efficient         but lighter turbo-shaft range extender with higher density         batteries, offering best-in-class regional speeds but at         modestly higher operating costs;     -   The powertrain operation may provide optimal efficiency over         regional ranges via maximized use of the rechargeable storage         units; this may be implemented by targeting full depletion         during a flight (or lower if within electric range (A)), with         range-extending generators switched on only if the available         stored energy is insufficient to complete the flight. This         translates to ultra-efficient stored energy flights within the         electric-only range (A), and very efficient hybrid flights over         the longer hybrid range (B) or total range (C);     -   Safety reserves are maintained across the rechargeable storage         units and the range-extending generators to maximize use of the         rechargeable storage units. For example, if the power output of         the onboard range-extending generators enables safe maneuvering         of the aircraft, then reserves are maintained as fuel for the         generators sufficient for operation for a length of time         determined by regulation or by other means. If range-extending         generators do not enable safe maneuvering of the aircraft, then         the fuel reserves are supplemented by stored energy equivalent         to that required to enable maneuvering over the target length of         time;     -   The powertrain is “plug-in” and the rechargeable storage units         are designed to be replenished by ground-based charging         stations. This is enabled by an onboard charging platform for         connecting to the mains or to a fast charge station for low or         high-rate recharge in-situ. Also included are fast swap         capabilities for the rechargeable storage units via release         mechanisms to enable quick replacement of depleted units with         charged ones;     -   Each rechargeable storage unit may be comprised of multiple         individually swappable modules. This enables increased         efficiency on low payload flights by loading additional modules         to extend the electric-only range. Or unloading some modules to         increase payload capacity, but at loss of electric-only range.         In case of batteries, this is enabled by design of cell modules         that plug into bays within a battery pack. Each module may         contain one or many cells with wiring, sensors and controllers,         along with first-level cooling, structural support and fire         protection features. Easy installation is enabled by connectors         to the battery pack power, sensor, control and cooling circuits,         and by quick release mechanisms; and     -   The powertrain design provides energy recovery via regenerative         braking of the propulsors. To enable this, propulsors are         equipped for varying degrees of air braking through use of a         variable pitch propeller or another mechanism, e.g., adjustable         exhaust plug. As a result, the rechargeable energy storage         receives energy from electric motors operating as a generator         when air braking is engaged. The powertrain is also designed for         selective charging by the range-extending generator during low         power operations. In this mode, some or all of the electrical         energy generated by the range-extending generator is directed to         the rechargeable energy storage units.

The inventive powertrain designs and configurations are architected to permit graceful degradation, for safety and fault tolerance exceeding stringent aviation requirements (FAA and EASA). This includes ability to tolerate failures in power sources (energy storage units, generators), motors (propulsion, generator), convertors (inverters, rectifiers, DC-DC convertors), distribution (buses. wiring), controls (sensors, communication), as well as safety in event of moderate or severe impact on the system.

To achieve this, the powertrain is designed for graceful degradation, whereby failure in any area has no more than a fractional impact on the performance of the powertrain, allowing near normal flight to a nearby airport for repair. At least three unique aspects of the inventive hybrid powertrain enable this, with only a modest cost or weight penalty:

-   -   Multiple power sources on-board create a simple path to graceful         degradation, by the sizing of sources so the aircraft is able to         fly on only a subset of these;     -   The ability to design the powertrain with multiple fractional         components, each with high peak-to-continuous performance,         limits the impact of failures to less than an equivalent         fraction of the function. Electric components, e.g., motors,         converters, distribution buses, wiring, switches, allow this         with only modest cost or weight penalties, unlike mechanical or         hydraulic components. Many of these also come with have high         peak to continuous performance capabilities (often heat limited)         so that surviving components can compensate to some degree for         failures in others during recovery periods; and     -   High-speed solid-state sensors and connectors enable detection         and remediation of failures within milliseconds in contrast to         microseconds for traditional contactors or even seconds for         mechanical devices. As a result, embodiments of the inventive         hybrid powertrain are uniquely able to engage redundant         components and redistribute power to surviving components, on a         timescale comparable to the physical.

In some embodiments, a design for graceful degradation includes sizing the power sources, rechargeable energy storage units and range-extending generators so that the aircraft can maneuver safely in the event of a failure of one or multiple of these elements. For example, the aircraft may be designed to fly on the rechargeable storage units or range-extending generators alone, for tolerance to failure in any one. Moreover, a multiplicity of storage units or generators may be used for further safety to reduce the likelihood of a complete loss of the source. This design of power sources is combined with distribution elements (e.g., buses, switches, and wiring) architected to re-route power in the event of a fault (as illustrated in FIG. 25), so propulsors receive equitable distribution from the surviving sources. This re-routing is managed by the powertrain optimization and control system (POCS). Failures of the storage units or generators are detected by a fault-detection and recovery module of POCS, which then redistributes power optimally to maintain safe flight. In addition, POCS also ensures storage units and the fuel system retain sufficient reserves to independently meet safety requirements.

A design for graceful degradation may also include use of multiple fractional components, propulsors, generators, motors and storage units, for fault tolerance against failure in any one. This may include powering the powertrain with more than one propulsor or generator, and powering each with more than one motor, so failure in any one component does not equate to loss of the entire capability. Individual motors may be designed for peak performance 60-80% above continuous for recovery periods of 5-10 minutes, so surviving motors are able to power up to compensate for motor faults in others. This peak output capability is combined with distribution (buses, switches, wiring) architected to re-route power to surviving motors to enable them to peak safely. Failures of propulsors, generators, motors or storage units are detected by the Fault-detection and recovery module of POCS, which then redistributes power optimally to maintain safe flight.

A design for graceful degradation may also include architecting the distribution elements (e.g., buses, switches, wiring, fault isolating components) with redundancy so that the powertrain is resistant to faults in individual circuits. This may include use of multiple buses, each feeding one or more propulsors, along with back-up buses so that the impact of a bus fault is limited to a subset of the propulsors, and so that power to the impacted propulsors can be re-routed via a redundant bus. This bus architecture is combined with wiring and switches so that power from the sources is distributed equitably to primary and back-up buses, and so that power to propulsors can be routed via the primary or back-up bus. This may also include fault tolerant schemes for converters, e.g., redundant converters, or redundant phase legs, with fault isolation, so functionality of a faulty converter is largely recovered. Failures of the distribution system are detected by the Fault-detection and recovery module of POCS, which then redistributes power optimally to maintain safe flight.

A design for graceful degradation may also include design of the powertrain control system (POCS) so that it is able to operate safely in the event of a failure of one or more sensors. This may include sensor fault detection capability in the fault-detection and recovery module within POCS, and back-up sensors or sensor-less (sensor independent) monitoring to cover critical sensor failure modes. For instance, propulsor motor fault tolerant control is managed by the fault-detection and recovery module within POCS that monitors flight conditions to detect and diagnose issues, and then redistributes power to the healthy motors in an optimal way to restore sufficient flight capabilities.

Also included in the inventive design(s) are procedures for safety in the event of a crash. For example, the fault-detection and recovery module within POCS triggers the emergency isolation of high-voltage circuits, e.g., storage units, generators, convertors, on pilot request or when significant impact is detected. Note that the graceful degradation measures mentioned are coupled with the distribution architecture to reroute power with a minimal impact on performance in the event of a failure. For example, FIG. 8 shows the architecture for a representative twin propulsor aircraft with two rechargeable storage units and a single range-extending generator, implemented using a redundant vital bus.

FIG. 26 is a diagram illustrating an example user interface 900 for use by a pilot of an embodiment of the inventive aircraft. The figure shows various operational and status indicators and may be used in an embodiment of an electric-hybrid aircraft that is part of the inventive air transportation system. In one embodiment, the displays are digital and represent performance parameters in the same or a similar format to those of a conventional aircraft for ease of use. The figure illustrates an example of the pilot interface in the “Inflight optimization and control” mode of operation and contains the following indicators and information:

-   -   Color coding is chosen to be industry typical to ease pilot         transition. Items in green or white are labels; items in magenta         are active indications of system status. Triangle “bugs” show         either a current indication, or a labeled target indication.         Color coding uses standard green/yellow/red for         normal/caution/danger operating zones;     -   Power indicators (upper left) show the current propulsor power         output both in RPM and % maximum power, as commanded by the         power lever. These are very similar to conventional gas turbine         power output indicators;     -   Speed tape showing current vs. target airspeed (upper right)         utilizes an industry standard vertical airspeed indicator in         units of knots indicated airspeed, KIAS. Specific to this         innovation, shows “speed bugs” at the calculated speeds to fly         for one or more flight modes; in this example, showing “High” at         213 KIAS, “Optimal” at 196 KIAS and ECON is indicated without a         bug to show that it's below the current range of the speed tape;     -   The second row of indicators shows battery, fuel, and power         balance. Battery and fuel are shown with the industry typical         indicators including color coding for normal, caution, and         exhausted energy states. When coupled with an active flight plan         through POCS, an “energy bug” is enabled which shows the         expected energy state on landing (shown on both battery and         fuel). The split pie-chart shows the balance of generation power         to battery power; this is a unique indicator to hybrid-electric         aircraft;     -   The lower quadrants show more detailed data on powertrain         systems and is configured for the current powertrain components.         The example shown here utilizes three discrete packs of         batteries, coupled with a turbo-diesel reciprocating generation         engine; pertinent information for each is displayed using         typical indicator styles. These lower quadrants may display         multiple system information pages, with the pilot able to scroll         through the information. These displays are specific to the         hybrid-electric powertrain implementation; and     -   This cockpit interface to the hybrid-electric powertrain has         multiple modes; in this example mode selection is through a         three position knob on the bottom right. The “flight” mode is         shown here; additional modes could be “Calibration”, invoked         each time a module changes, “Pre-flight” which would initiate         and display status on the internal systems self checks flight,         and “diagnostics” which could display more detailed information         on all systems monitored and controlled, primarily used for         system configuration, maintenance and repair.

In addition to the display shown 900 in the figure and the associated aircraft functions or systems, the underlying powertrain optimization and control system (POCS) platform may permit control of one or more specific powertrain capabilities, including but not limited to the rechargeable energy storage units (e.g., batteries, super-capacitors, and range-extending generators), internal combustion engines, or fuel cells. POCS offers a unified interface to the modules of the powertrain, to simplify installation, flight preparation, flight operation and diagnostics.

The capabilities of POCS are important to the early adoption of hybrid-electric aircraft, by optimizing operation for maximum efficiency over regional flights, via quick and safe remediation of faults, by reducing pilot workload and easing pilot transition to electric powertrain, and by simplifying module changes to alternatives or future technologies. Embodiments of the POCS may assist in the adoption of hybrid-electric aircraft based regional air transportation systems as a result of one or more of the following:

-   -   Enables range-optimized regional flights by optimizing sources         of energy over a flight path. To maximize efficiency, energy         sourcing should prioritize the lower cost source, typically the         energy storage units, over the higher cost sources, typically         the generators, over the course of a flight. For instance,         flights over ranges longer than the electric-only range should         deplete the lower cost energy storage units to a minimum         permissible level determined by safety or battery life         considerations. Moreover, sourcing should be charge blended,         utilizing both storage units and generators optimally throughout         the journey, while ensuring energy draw promotes safety and         operating life. POCS enables this by determining an optimal         Energy plan that minimizes the total cost of the flight         (operator defined) within system constraints, based on the         flight path and Flight mode, the departure and arrival Energy         States and characteristics of the aircraft. This defines the         Energy state of the hybrid powertrain along the path to the         destination, e.g., percent state of charge of the battery packs,         percent fuel capacity for the generators, and guides the real         time flow of power from the storage units and generators. POCS         enables further optimization by identifying opportunities to         upsize the energy storage units on low payload flights that are         longer than the electric-only range;     -   Optimally controls the real-time flow of power from the storage         units and generators to achieve the targeted Energy plan.         Although the Energy plan defines an overall sourcing strategy         for the flight, this is inadequate for real-time control given         need to accommodate unpredictable, varying flight environments.         Further, there is need to direct each of the powertrain modules         to deliver the requested power in an optimal way, e.g.,         generator operating on its optimal working curve. POCS enables         this in two stages. First, by determining optimal sourcing for         the power requested that aligns with the Energy plan, defining         real-time power flows from the energy storage units and the         generators. Second, settings of the powertrain modules are         optimized to deliver the requested power with maximized         efficiency, and these are used to direct the module controllers.         For instance, the propulsion power requested is delivered via         optimized settings for the propulsion motors (torque, RPM) and         the propulsor (e.g., fan pitch angle, exhaust plug position).         POCS also manages energy harvesting during the flight, for         instance, via regenerative braking of the propulsors, or via the         generators during low power operations;     -   Enables fault-tolerant control of the powertrain: assists the         operator with preserving normal or gracefully degraded operation         in the event of faults. The hybrid powertrain is designed for         graceful degradation, whereby failure in any area has not more         than a fractional impact on the performance of the powertrain.         This is enabled by multiple power sources onboard, by design         with multiple fractional components, by use of redundant         components and circuitry, and by use of high-speed solid-state         sensors and connectors for quick detection and remediation. POCS         builds on this capability by enabling quick assisted response to         faults for continued safe flight. This is done by continuous         monitoring of the health of the powertrain by the Fault         detection and identification function. A combination of signals         and models are leveraged to identify and isolate faults as         quickly and accurately as possible. If a fault occurs, POCS         alerts the operator to trigger remedial response. POCS may also         trigger a redesign of the Powertrain for graceful accommodation         of the fault, and a redesign of the Controller to adjust to the         potentially redesigned powertrain with fault. The redesign of         the Powertrain and Controls may also be triggered by the         operator. POCS also assists with ensuring safety in event of a         crash, by isolating high-voltage circuits on pilot request, or         when impact is detected;     -   Offers a simplified, unified interface to the hybrid powertrain,         reducing pilot workload and easing pilot transition from         conventional to hybrid aircraft. Key to quick adoption of hybrid         propulsion is ensuring pilots can operate the more complex         powertrain with minimal incremental training. POCS enables this         via optimization and controls that shield the pilot from the         added complexity of the powertrain, coupled with an interface         that mimics those of conventional aircraft. In addition, POCS         offers automation to streamline a range of pilot and maintenance         activities, such as powertrain calibration, pre-flight         preparations, inflight control, and powertrain diagnostics; and     -   Streamlines installation of new modules to support forward         compatibility and modularity. This key differentiation of hybrid         powertrain is enabled by POCS in two ways. First, POCS offers         standardized control and monitoring interfaces to a range of         module alternatives, allowing a switchover of generators,         upgrades to advanced technology energy storage units, or the         addition or removal or storage modules to compensate for         payload. Second, POCS enables simple calibration to new modules         via upgrade of aircraft and powertrain models to match, operator         input or from online library, coupled with targeted performance         testing to fine-tune the model to the unit installed. Similar         testing is also performed on a routine basis to ensure models         maintain fidelity as modules age via use.

As will be described with reference to FIG. 27, POCS offers two interfaces to the operator, “integrated” and “modular”. Integrated is a simplified user interface to the powertrain that mimics the controls of conventional aircraft, reducing pilot workload and easing transition from conventional to hybrid. Modular is a direct user interface to individual modules of the powertrain, enabling lower-level fine control of the operation of the powertrain. These are described in greater detail below:

-   -   Integrated. Unified front-end to all POCS capabilities, enabling         the operator to toggle to appropriate operating mode:         Calibration; Pre-flight; Inflight control; Diagnostics. Displays         use performance parameters similar to that used in conventional         aircraft to ease pilot transition to the new technologies and         capabilities. An example of the pilot interface in the “Flight”         mode is shown as display 900 in FIG. 26. The display is coupled         with cockpit controls that are similar to those found in         conventional aircraft today, and which translate operator input         to optimal settings of the underlying hybrid powertrain based on         the defined objective function for the flight, such as:         -   Power levers—one for each propulsor, controlling the power             output of the propulsor. The Power lever angle (PLA)             determines the power output of each propulsor as a percent             of Full power, and enables limited duration surges to Peak             power. The Inflight control module converts power requested             by the Power levers to optimal settings of each propulsor in             real-time, and sources this power optimally from onboard             generators and stored energy units to meet the operator             defined objectives for the flights, within constraints of             the aircraft and powertrain. Some embodiments may also offer             Fan or propeller control levers to control the RPM of each             propulsor, ranging from Maximum to Minimum, with extension             to Feathering. Some embodiments may allow controls of             multiple propulsors to be synchronized, so all coupled             propulsors operate with the same settings, or allow             auto-throttle operation, whereby the Inflight control module             directs the throttle based on the Flight path. In these             situations, a servo motor or similar mechanism is used to             move throttles based on the current power setting (standard             FAA auto-throttle operation).         -   Regenerative braking controls—one for each propulsor,             controlling the regenerating power output of the propulsor.             This is done via dedicated Regenerative braking levers, or             by extending the range of the Power levers to negative power             levels, extending from zero power to full regenerative             power. In both cases, the lever angle determines the             regenerative power output of each propulsor as a percent of             full regenerative power. The Inflight control module             converts regenerative power requested by the levers to             optimal settings of each propulsor in real-time.         -   Reverse power controls—one for each propulsor, controlling             the reverse power output of the propulsor. This may be done             via dedicated Reverse power levers, or by extending the             range of the Power levers to negative power levels,             extending from zero power to full reverse power. In both             cases, the lever angle determines the reverse power output             of each propulsor as a percent of full reverse power. The             Inflight control module converts reverse power requested by             the levers to optimal settings of each propulsor in             real-time.         -   Lower-level controls—offered in some embodiments enable             operators to control the Generators, Storage units and Power             distribution manually. These could include Generator             controls for switching generators on and off, control             generator output from idling to peak power. Controls could             also include a Power distribution control to redirect power             flow from the onboard sources to the propulsor. In a twin             propulsor aircraft with left-right propulsors and left-right             sources, these could offer a choice of Split flows (left to             left, right to right), Directional (Split plus right to left             or left to right) and Coupled (left-right to left-right);             and     -   Modular. Offers a direct interface to individual modules of the         powertrain via their onboard controllers. Intended for         situations where there is need to bypass the flight optimization         capabilities of POCS to engage the controllers directly, e.g.,         repair, emergencies, non-standard operations. The range of         modules that may be accessible for a sample hybrid powertrain is         shown in FIG. 27, and described in greater detail herein.

FIG. 27 is a diagram illustrating the primary functional elements or modules of a powertrain optimization and control system (POCS) that may be used in an embodiment of an electric-hybrid aircraft that may be used as part of the inventive air transportation system. Each or a combination of the functions, operations, or processes performed by or under the control of the elements or modules shown in the figure may be performed by the execution of a set of instructions by a properly programmed processing element (such as a controller, state system, microcontroller, CPU, microprocessor, etc.).

As shown in the figure, the elements or functional modules of an embodiment of a POCS platform 1000 may include an “onboard” component 1002 and an “online” component 1004. The onboard components, elements, and processes 1002 are typically resident on the aircraft being controlled, while the online components, elements, and processes 1004 are typically resident on a data processing platform or system that is located remotely from the aircraft (such as in a control center, centralized data processing and scheduling platform, etc.) and which communicates with the onboard component 1002 (when necessary) via a suitable communication channel or combination of communication channels (such as a wireless technology coupled to a server that may be communicated with via the Internet).

In an exemplary embodiment, the POCS platform 1000 functionality is enabled by the following onboard capabilities (components, elements, and processes 1002):

-   -   Standard procedures (element/component 1041), which is a library         of preset and operator defined standard operating procedures for         the powertrain and its modules, and may include:         -   Flight mode(s): For instance, Optimal, High speed, Economy,             Custom;         -   Scans and diagnostics: For instance, Initialization scan,             Energy scan, Pre-flight scan, Inflight scan, Post-flight             scan;         -   Operating rules library: Defines operating priorities for             the powertrain, required by safety or based on operator             preference. These constrain the Hybrid energy planner and             Hybrid power manager, and may include:             -   Minimum energy state to ensure adequate safety reserves,                 for instance, stored energy units at 20% of capacity,                 and generator fuel equivalent to 45 min of flight;             -   Energy state at arrival, for instance, to deplete stored                 energy units to minimum levels at 20% of capacity;             -   Power distribution priorities by flight leg. For                 instance, taxi on stored energy alone, or approach on                 stored energy alone, and with generator on idle for high                 availability; and             -   Power level settings by flight leg. For instance, climb                 at 80% full power, or for initial descent to be at                 neutral thrust.     -   Onboard logs. A database capturing information on key aspects of         the powertrain and its performance. These include: operator         details; onboard modules; operator preferences; lifecycle and         maintenance records; performance logs; checks and diagnostics         logs, access history logs. The database transmits logs to the         Online log (element/component 1023) via the secure datalink         (element/component 1043) periodically, and only stores limited         history onboard.     -   Secure Datalink (element/component 1043). This enables periodic         uploads from Onboard logs to the Online log(s) 1023 for the         specific powertrain, remote diagnostics and maintenance of the         powertrain, and accessing the aircraft and powertrain library         (element/component 1020) for calibration or benchmarking         purposes. The datalink may include 2-levels of security, a lower         level for communicating log or library data, and a higher level         for diagnostics and maintenance data. Access to the datalink is         secure and all access history is logged. This also enables         two-way data flow between POCS and FPOP/FMS for flight data.     -   Module interfaces (element/component 1050). These are connector         interfaces to lower-level controllers of the range of modules         onboard, enabling the controllers to be queried or directed by         the POCS platform, and to transmit a range of state and         performance information to POCS on-demand or continuously.         Typically, API specifications define the protocols by which POCS         and modules communicate. Control modules may include variable         pitch fan controllers, propulsor motor controllers, battery         management systems, engine controllers, fuel system controllers,         generator motor controllers, distribution controllers (switches,         connectors, and convertors), etc.         Note that FIG. 29 shows the interface configuration for an         example powertrain 1200 coupled to the POCS onboard by several         interfaces/connectors 1202 for purposes of sensing a performance         parameter and returning a control signal to a component of the         powertrain or its control system. Similarly, FIG. 27 and FIG. 28         show the communications and data flow from the POCS Optimization         Modules 1130 and Powertrain and Controls Manager 1142 to         matching module interfaces (elements 1050 and 1150) of the         powertrain. The system offers the pilot a secondary direct path         (element 1052) to module controllers via the Modular operator         interface (element/component 1012). The system may include         back-up connectors to module controllers for redundancy, along         with connectors and switches to activate the back-up circuitry.

The POCS platform 1000 may also provide one or more of the following online capabilities via a secure POCS cloud-based data platform (element/component 1004):

-   -   Aircraft and powertrain library (element/component 1020). A         library of aircraft and module performance models, including         operating models for each class, and parameters for each module         in a class. The models and parameters are updated periodically         via the benchmarking platform (element/component 1021). The         database in designed to be queried by onboard POCS when         initializing, calibrating a new module, or for periodic         refreshes.     -   Benchmarking platform (element/component 1021). This is a         database of performance benchmarks, and may include input         capability for external benchmarks, and upload of raw         performance data from the online logs for individual         powertrains. Also includes statistical or other data analysis         procedures that update benchmarks periodically.     -   Diagnostics platform (element/component 1022). This has the         capability of enabling remote diagnostics and maintenance of the         powertrain over a highly secure datalink.     -   Powertrain logs (element/component 1023). This serves as an         archive of Onboard logs from individual powertrains uploaded         periodically via the secure datalink, coupled with comparative         performance statistics drawn from the Benchmarking platform.

In a typical embodiment, an implementation of a POCS platform (such as element 1000 of FIG. 27) may offer the following functionality or capabilities:

-   -   Calibration (represented by element/component 1025). This         tailors the optimization and control platform to accommodate         specific modules onboard, enabling upgrades to future         technologies, aircraft variants, up/downsizing energy storage         units or generators, and high fidelity modeling of module         performance. In some embodiments it may perform or assist in         performing the following functions/operations:         -   Scan powertrain to identify changes to modules onboard             relative to most recent scan, validate and record in Onboard             log;         -   Download optimization and control parameters for all changed             modules from the Online Aircraft and Powertrain library via             the datalink and populate onboard models;         -   Calibrate models to performance of onboard modules. Step             operator through a series of tests on the modules, defined             by the Initialization scan procedure to assess actual versus             model performance. Identify potential issues and alert.             Adjust model parameters to better match actual performance;             and         -   Enable operator to define range of preferences around             powertrain optimization and control, displays, reporting,             monitoring and diagnostics. Includes setting tailored to the             specific operating environment, mission profiles and             trade-offs. Store preferences in Onboard log.     -   Flight prep (represented by element/component 1026). This         performs automated checks to ensure the powertrain has adequate         energy to safely complete the planned flight, and is in flight         ready state. In some embodiments it may perform or assist in         performing the following functions/operations:         -   Accept Flight mode or Optimal as default, and flight             details: Air path or Flight time (or distance), Payload and             an Uncertainty factor. These can be entered manually or via             FPOP;         -   Calculate and display Energy state based on Energy state             scan procedure;         -   Determine if additional generator fuel or stored energy is             required by using the Hybrid Energy Planner with prescribed             flight details, Flight mode and Energy state. Examine             Payload versus design payload to ensure safe flight, and to             assess if there is option to increase capacity of stored             energy units;         -   If onboard energy or Payload is changed, via additional             stored energy units, increased state of charge or fuel,             re-run Hybrid Energy Planner; and         -   Execute pre-flight tests on the powertrain defined by             Operating procedures, and identify issues, trigger the Fault             detection and recovery module.     -   Inflight Control (represented by element/component 1027). This         enables simplified control of power delivered by the hybrid         powertrain, optimizing powertrain and module performance based         on operator selected Flight mode and flight details. Control may         be semi- or fully-automated, and optimization may be Basic or         Integrated. In some embodiments it may perform or assist in         performing the following functions/operations:         -   Calculate the optimal Energy plan, Arrival energy state,             Target speeds and ranges (Maximum, Optimal, and Economy)             using the Hybrid energy planner based on flight details,             Flight mode and Energy state Flight details are either             Basic, with Flight time (or distance) as input, or             Integrated, with Air path as input. Display Arrival energy             state and Target speeds. The Energy plan describes the             Energy state along the Air path: Stored energy and Generator             fuel at discrete waypoints;         -   If Arrival energy state is below minimum reserve levels,             alert the operator, and offer alternate Flight mode, Target             speed settings to make the destination;         -   Control the powertrain in real-time to achieve the Energy             plan, in tandem with operator input, optimally sourcing             power from the storage unit and generators, making             adjustments as conditions change. FIG. 28 provides an             overview of the Inflight control process and the functional             modules or sub-processes that may be utilized, and is             described in greater detail herein. This control process may             include the following functions or operations:         -   Determine real-time control strategy using the Hybrid power             manager, defining optimal energy distribution across the             generators and stored energy units;         -   Pass the energy distribution to the module optimizers which             compute optimal settings for the powertrain modules, and             transmit these to the lower-level module controllers via             Module interfaces within POCS;         -   Refresh Energy plan periodically based on deviation from             prior plan;         -   Refresh Energy state periodically via the Energy state scan             procedure; and         -   Enable updates to Flight mode and flight details manually or             via FPOP (described with reference to FIG. 31), and respond             by refreshing the Energy plan.     -   Enable semi-automated or fully-automated operation: the operator         controls the Power levers in the former; the Inflight control         module directs all functions in the latter, adjusting requested         power levels to deliver the optimal airspeed; and     -   Continuously monitor the performance of the powertrain via the         module optimizers, and assess against models, safe limits via         Inflight scan procedure. In event of issue, trigger the Fault         detection and recovery module to coordinate alerts and action.

Diagnostics (represented by element/component 1028). This performs post-flight mission analysis, powertrain diagnostics and issue resolution. In some embodiments it may perform or assist in performing the following functions/operations:

-   -   Run the Mission analysis algorithm on monitored data stored in         the Onboard log to calculate and display key flight statistics         (e.g., distance, time, average speed), detail on total energy         used, fuel and stored energy remaining, key performance         statistics (e.g., overall efficiency and by module). Store         results in Onboard log;     -   Update operating history of modules or components that require         periodic maintenance or are life-limited; and     -   Monitor health and performance of the powertrain, and assess         against models, safe limits via Post-flight scan procedure. In         event of issue trigger the Fault detection and recovery module         to coordinate alerts and action.

Fault detection and Recovery (represented by element/component 1042). This performs ongoing monitoring of the powertrain to detect and identify faults, alert the operator, and assist with recovery action. In some embodiments it may perform or assist in performing the following functions/operations:

-   -   Monitor the health of the powertrain by the Fault detection and         identification function, leveraging a combination of signals and         models to identify and isolate faults as quickly and accurately         as possible;     -   If a fault occurs, alert the pilot to trigger remedial response         via the Powertrain alert function;     -   If a fault occurs, determine action required and trigger the         Powertrain and Controls manager to execute in concert with the         pilot; and     -   Remedial action may also be initiated by the pilot engaging the         Powertrain and Controls manager to execute.

In some embodiments, the POCS determines an optimal Power plan based on flight details and a prescribed Flight mode. POCS then controls the operation of the powertrain and its modules during flight to match the Power plan by monitoring performance of the powertrain and modules, making adjustments when necessary. POCS is designed for semi-automated or fully-automated operation, with the pilot retaining control of the throttle in the former, while POCS controls all functions in the latter. However, the pilot is able to override POCS settings.

FIG. 28 is a diagram illustrating the primary functional elements or modules of a POCS that may be accessed and used to control or modify on-aircraft processes in an embodiment of the inventive air transportation system. Each or a combination of the functions, operations, or processes performed by or under the control of the elements or modules shown in the figure may be performed by the execution of a set of instructions by a properly programmed processing element (such as a controller, state system, microcontroller, CPU, microprocessor, etc.).

As shown in the figure, the elements or functional modules of the on-aircraft processes of an embodiment of a POCS 1100 may include:

Optimization Modules (Element 1030 of FIG. 27 and/or Element 1130 of FIG. 28)

Hybrid Energy Planner (Element 1132 of FIG. 28):

-   -   Determines the optimal Energy path over an Air/flight path by         minimizing a non-linear cost objective (see below) subject to         initial and arrival Energy states, Module performance         constraints and Operating rules, typically a charge blended         strategy that looks to gradually deplete the stored energy units         over the course of the flight, enabling each of the power         sources onboard to operate with optimal efficiency;     -   Performed by decomposing the trip into an Air path, composed of         segments with roughly uniform operating requirements, e.g.,         taxi, take-off roll, climb at uniform rate, cruise, power         neutral descent (done as part of Flight prep). Optimization is         then performed to determine the optimal Energy plan along the         provided Air path. If a detailed Air path is not provided, a         standard depletion profile is assumed, for instance, linear over         the cruise and climb legs, after budgeting for taxi, take-off,         descent and landing based on a look-up table of benchmarks;     -   Optimization is performed via full dynamic programming (or a         similar algorithm), or simplified approaches such as using         look-up tables or functions to determine optimal power         distribution across the generators and the stored energy units         based on the flight leg and operating conditions. Power         distribution could be described in one of many ways, including         the Generator power setting, as fraction of full generator         power, or the Power ratio, equal to the ratio of power drawn         from stored energy to the total power requested;     -   The objective function defines the quantity to the minimized         over the course of the Air path by the Hybrid energy planner.         For example, the objective function may include one or several         of the terms below, with parameters defined by the operator:         Objective function=Cost of fuel+Cost of stored energy+Cost of         engine maintenance and reserves (amortized)+Cost of battery         packs (amortized)+Cost of passenger and crew time+Cost of         aircraft+Cost of emissions; and     -   The objective function is minimized based on provided departure         and arrival Energy states, Operating rules from the Operating         rules library, powertrain and module performance constraints         from the powertrain and module (propulsor, generator, stored         energy) models. The optimization process requires simulation of         aircraft and powertrain performance, provided by the Aircraft         and Powertrain performance models.

Hybrid Power Manager (Element 1134 of FIG. 28):

-   -   Determines real-time control strategy to optimize energy         distribution, described via variables such as the Generator         power setting or Power ratio, across onboard sources to deliver         the power demanded, based on the overall Energy path for the         flight. This is determined by minimizing a non-linear objective         for the flight segment subject to Module performance constraints         and Operating rules, including the provided Energy path;     -   Optimization may be performed by a relatively simple method,         such as determining the Generator power setting (or Power ratio)         from a look-up table of optimal values by flight leg and         discrete range of operating conditions, or by a more complicated         method, such as determining the optimal value using one of         several algorithms such as Pontryagin's minimum principle (PMP)         or the Equivalent consumption minimization strategy (ECMS).         Alignment with the provided Energy plan is driven by an outer         control loop, e.g., proportional plus integral;     -   The objective function defines the quantity to the minimized         over the course of the Air path by the Hybrid energy planner.         For example, the objective function may include one or several         of the terms below, with parameters defined by the operator:         Objective function=Cost of fuel+Cost of stored energy+Cost of         engine maintenance and reserves (amortized)+Cost of battery         packs (amortized)+Cost of passenger and crew time+Cost of         aircraft+Cost of emissions; and     -   The objective function is minimized based on provided departure         and arrival Energy states, Operating rules from the Operating         rules library, powertrain and module performance constraints         from the powertrain and module (propulsor, generator, stored         energy) models. The optimization process requires simulation of         aircraft and powertrain performance, provided by the Aircraft         and Powertrain performance models.

Propulsor Optimizer (Element 1136 of FIG. 28):

-   -   Determines real-time control strategy for each of the propulsors         based on power requested, airspeed and environmental conditions.         Translates power requested to propulsor settings for optimal         efficiency. The optimal settings are then used to direct the         lower-level module controllers (e.g., variable pitch fan         controller, motor controller) via Module Interfaces (elements         1050 of FIG. 27 and/or elements 1150 of FIG. 28) within the POCS         platform. The optimal settings may be further modified via a         fine control loop for improved performance. This may include a         peak seeking loop to fine-tune the operating point, and a         smoothing loop to moderate abrupt changes in settings over         intervals determined by ride comfort, aircraft structural or         powertrain performance constraints;     -   Settings optimized may include the attitude of the propulsors,         e.g., variable pitch fan angles, position of the exhaust plugs,         and the output of the propulsor motor-inverters, e.g., torque,         speed. The optimizer regulates propulsor setting over the range         of aircraft operations including standard thrust, neutral         thrust, regenerative braking, reverse thrust and recovery         thrust;     -   For standard thrust control, the optimizer is driven by         requested power for each propulsor, airspeed and environmental         conditions, and determines propulsor settings that maximize the         thrust delivered. This is done by performing a staged or coupled         optimization across the Motor and the Propulsor performance         models (element 1040 of FIG. 27 and/or element 1140 of FIG. 28).         In the staged approach, motor and propulsor settings are         optimized in turn. For instance, motor settings to maximize         efficiency may be determined first, by optimization using the         Motor performance model within operating constraints defined for         the motor. Following this, the propulsor attitude is determined,         e.g., fan pitch angle or position of exhaust plug, to maximize         propulsor thrust by optimization using the Propulsor performance         model within operating constraints defined for propulsor         attitude. These settings are then used to direct the operation         of the lower level controllers via the Module Interfaces within         POCS;     -   In some implementations, a look-up table may be used to         determine near-optimal values, followed by an optional         optimization step to refine estimates along the lines of those         described above;     -   For reverse thrust, the optimizer is driven by requested reverse         power for each propulsor, and determines propulsor settings that         maximize the reverse thrust delivered. This is done by process         similar to that for standard thrust. In the case of a staged         approach, the motor settings are determined to maximize         efficiency, by optimization using the Motor performance model,         or by look-up in the Motor performance table. Then the matching         propulsor attitude is determined by optimization using the         Propulsor performance model or by look-up in the Propulsor         performance table;     -   For regenerative braking thrust control, the optimizer is driven         by requested reverse power for each propulsor, airspeed and         environmental conditions, and determines propulsor settings that         maximize the thrust delivered. This is done by performing a         staged or coupled optimization across the Motor and the         Propulsor performance models. In the staged approach, motor and         propulsor settings are optimized in turn. For instance, motor         settings to maximize efficiency may be determined first, by         optimization using the Motor performance model within operating         constraints defined for the motor. Following this, the propulsor         attitude is determined, e.g., fan pitch angle or position of         exhaust plug, to maximize propulsor reverse thrust by         optimization using the Propulsor performance model within         operating constraints defined for propulsor attitude. These         settings are then used to direct the operation of the lower         level controllers via the Module Interfaces within POCS; and     -   For neutral thrust, the optimizer directs the lower-level         controllers to cut off power to the motors, and set the         propulsor attitude to minimal drag, e.g., variable pitch fans         set to feathering or pin-wheeling, exhaust plug set to maximum         extension.

Stored Energy Optimizer (Element 1138 of FIG. 28):

-   -   Monitors performance and state of the rechargeable storage units         via the Module Interfaces 1150 within POCS, to help ensure         operation within long life ranges defined by performance         constraints. If the storage units are outside of their long life         ranges, the optimizer adjusts the hybrid energy planner and         hybrid power manager settings to redistribute power to the         generators, e.g., by increasing the effective cost of the         storage units;

Generator Optimizer (Element 1140 of FIG. 28):

-   -   Determines real-time control strategy for each of the generators         based on power requested, airspeed and environmental conditions.         Translates power requested to generator settings for optimal         efficiency. The optimal settings are then used to direct the         lower-level module controllers (e.g., engine control unit, motor         controller, fuel system controller) via Module Interfaces 1150         within the POCS platform. The optimal settings may be further         modified via a fine control loop for improved performance. This         may include a peak seeking loop to fine-tune the operating         point, and a smoothing loop to moderate abrupt changes in         settings over intervals determined by ride comfort, aircraft         structural or powertrain performance constraints;     -   Settings optimized may include the output of the internal         combustion engines, e.g., speed, torque, and the draw of the         generator motor-inverters, e.g., torque, speed;     -   The optimizer is driven by requested power for each generator,         airspeed and environmental conditions, and determines generator         settings that maximize efficiency. This is done by performing a         staged or coupled optimization across the Engine and Motor         performance models (or the integrated Generator model). In the         staged approach, engine and motor settings are optimized in         turn. For instance, engine settings to maximize efficiency may         be determined first, by optimization using the Engine         performance model within operating constraints defined for the         engines. Following this, the motor settings to maximize         efficiency are determined by optimization using the Motor         performance model within operating constraints defined for the         motors. These settings are then used to direct the operation of         the lower level controllers via the Module Interfaces within         POCS;     -   In some implementations, a look-up table may be used to         determine near-optimal values, followed by an optional         optimization step to refine estimates along the lines of those         described above; and     -   Includes strategy to route excess power (above that requested)         from the generators to charge the storage units to insulate         generators from transients, or when power requested is outside         of generator optimal range.

Powertrain and Controls Manager (Element 1142 of FIG. 28):

-   -   Driven by input from the pilot or the Powertrain and Controls         redesign function, executes diagnostics or resolution processes,         reconfigures the powertrain or changes the control laws;     -   Receives resolution or diagnostic process steps, powertrain         reconfiguration instructions, control laws from the pilot and         the Powertrain and Controls redesign function;     -   Resolves conflicting commands and follows safety procedures         defined in the operating rules function;     -   Executes the rationalized set of changes in a safe sequence by         directing the lower-level controllers via Module interfaces, and         by modifying the optimization modules, aircraft and powertrain         models; and     -   For instance, in event of an impending emergency landing, a         “secure and isolate” sequence triggered by the pilot immediately         before touchdown would direct the Powertrain and Controls         redesign function to shut down or isolate all high-voltage or         flammable systems of the powertrain to protect passengers and         cargo. Alternately, the Fault detection and identification         function may trigger the sequence based on assessment of a crash         from the aircraft state variables.

Power Allocator (Element 1144 of FIG. 28):

-   -   Determines power distribution across on-board propulsors based         on pilot direction and powertrain capabilities. This may         include:         -   Power allocation to coordinate multiple propulsors, for             instance balanced power to eliminate yaw moment, e.g.,             propulsors powered for zero moment about the aircraft center             of gravity, or propulsor power determined by power setting             of a master propulsor;         -   Power allocation to accommodate propulsor faults optimally,             preserving normal or gracefully degraded performance aligned             with requested power. For instance, allocation may boost             power to healthy propulsors to compensate for faults, while             limiting yaw moment, ensuring power is above minimum needed             to maintain safe flight for that flight leg, and does not             exceed constraints on the propulsors; and         -   Power allocation for directional control, by distributing             power to create a requested yaw moment.

Aircraft and Powertrain Models (Element 1040 of FIG. 27 and/or Element 1150 of FIG. 28)

Aircraft Performance Model (Element 1152 of FIG. 28):

-   -   a flight test calibrated, single degree of freedom, physics         based performance simulation model which calculates the expected         power required for the current phase of flight given the         aircraft weight, velocity, air temperature, and pressure, and         rate of climb or descent (a further description of this aspect         is found in the discussion of the FPOP system).

Powertrain and Propulsor Model(s) (Elements 1153 and 1154 of FIG. 28):

-   -   Performance models, look-up tables and performance constraints         that enable optimization of propulsor settings based on power         requested, airspeed and environmental conditions. These may         include one or more of the following:     -   Motor performance model, for instance, describing the efficiency         of the motor as a function of motor torque, speed and voltage;     -   Propulsor performance model, for instance, defining propulsor         thrust as a function of the fan pitch angle, torque, airspeed,         fan speed, and setting, standard, reverse or regenerative         braking;     -   Motor and propulsor performance look-up tables defining         performance relationship at discrete points, as replacement for         optimization, or for use to generate starting approximations;         and     -   Performance constraints on motor and propulsor settings. For         instance, fan pitch angle ranges for standard operation,         regenerative braking and reverse thrust.

Generator Model (Element 1156 of FIG. 28):

-   -   Performance models, look-up tables and performance constraints         that enable optimization of generator settings based on the         power requested, airspeed and environmental conditions. These         may Include one or more of the following:         -   Engine performance model, for instance, defining engine             efficiency as a function of the engine torque, speed, and             ambient conditions;         -   Motor performance model, for instance, describing the             efficiency of the motor as a function of motor torque, speed             and voltage;         -   Engine and motor performance look-up tables defining             performance relationships at discrete points, as replacement             for a performance model, or for use as starting             approximations;         -   Alternately, a generator motor for integrated engine-motor             performance, for instance, describing generator efficiency             as a function of torque, speed, voltage and ambient             conditions. Similar to above, the generator motor could be             replaced by or supplemented with generator performance             look-up tables defining performance relationships at             discrete points; and         -   Performance constraints on engine and motor, or integrated             generator settings. For instance, engine power ranges to             full power, boost and peak, motor power ranges to full             power, boost and peak, along with safe durations for boost             and peak.

Stored Energy Model (Element 1158 of FIG. 28):

-   -   Performance models, look-up tables and performance constraints         to enable optimization of power distribution across the         rechargeable stored energy units and generators, based on power         requested, current state of charge, environmental conditions.         These may include the following:         -   Draw down model for the stored energy units, which relates             the state of charge of the units to the current drawn, for             example, via generic Coulomb counting;         -   Stored energy performance model, that determines operating             efficiency of the units based on the power drawn, state of             charge, ambient conditions, and other factors. For instance,             a Rint type equivalent circuit model coupled with models for             key parameters, such as for the open circuit voltage as a             function of the state of charge and temperature;         -   Stored energy performance look-up tables defining             performance relationship at discrete points, as replacement             for optimization, or for use as starting approximations; and         -   Performance constraints on stored energy units, including             limits on the state of charge and power drawn from the             units.

Fault Detection and Recovery (Element 1042 of FIG. 27 and/or Element 1160 of FIG. 28)

Fault Detection and Identification (Element 1162 of FIG. 28):

-   -   Continually monitor the health of the powertrain by combination         of signal-based and model-based methods to detect sensor,         actuator, component or module faults;     -   Periodically sample signals from a range of sources, control         signals from POCS to the lower-level controllers, output signals         from the lower-level controllers, powertrain and module sensors,         aircraft state variables;     -   Monitor signals to ensure powertrain is operating within safe         limits defined by performance constraints. If safe limits are         exceeded, monitor the extent and duration of the spike to assess         severity of the issue;     -   Trigger powertrain alerts (element 1164 of FIG. 28) to notify         the pilot, via the in-cockpit interface (element 1170, and more         specifically, Pilot Alerts element 1172);     -   Leverage a variety of methods to detect faulty signals, e.g.,         Fourier analysis, limit checking;     -   Compare the performance of the powertrain, modules and         sub-systems with internal models for components or processes to         identify potential faults, via methods such as parameter         estimation or neural networks;     -   Determine the location and nature of the fault, via signals and         models using analytic and/or heuristic methods. Classify faults         based on location, type and severity; and     -   Trigger the Powertrain and Controls Redesign function/process         (element 1166 of FIG. 28) to initiate corrective action.         Note that for purposes of management of the aircraft and         transportation system, a powertrain configuration and control         law library may include the following information, data, or         processes:     -   A resolution process that describes the process steps to be         followed to resolve a fault with the powertrain;     -   Each powertrain configuration describes settings, e.g.,         switches, connectors, contactors, that collectively need to be         set to implement the architecture, along with process steps to         execute a safe reconfiguration, and the control law to operate         the reconfigured powertrain; and     -   Each control law describes optimization and control procedures         for the powertrain, including objective functions, operating         rules, powertrain and module performance constraints, and         aircraft and powertrain performance models.

Powertrain and Controls Redesign (Element 1166 of FIG. 28):

-   -   Determine powertrain and controls redesign required to         accommodate active faults such that normal or gracefully         degraded performance is preserved, by combining pre-defined         designs with in-flight synthesis;     -   Determine if corrective action is required based on location,         type and severity of faults;     -   Select resolution processes, powertrain configurations and         control laws to optimally accommodate a fault via an expert         system (or other decision process) that combines look-up within         libraries of pre-defined resolution processes, powertrain         configurations and control laws, with synthesis to tailor a         response to the specific condition. For example:         -   Isolation of faulty modules or circuits. For instance, in             the case of a short-switch fault in a converter, a             fast-acting fuse connected to the switch can be used:             clearing the fuse isolates the switch when faulted;         -   Redistribution of power to accommodate faults. For instance,             in a powertrain with twin propulsors, left and right, each             powered by a stored energy unit and generator, faults in a             propulsor or source may require allowing left-right             transfers to optimize output. A left to right transfer would             help compensate for faults in the left propulsor, by             allowing the right to be boosted. Similarly, a left to right             transfer would help accommodate faults in the right power             sources, so both can be powered equally;         -   Activation of redundant modules or circuits. For instance,             in the powertrain with twin propulsors, left and right, each             powered by a bus, failures in either of the buses can be             accommodated by a single redundant bus. Moreover, the             redundant bus can also be used to create a path for             left-right transfers; and         -   Isolation of high voltage circuits.     -   Initiate alerts and corrective action by triggering the         Powertrain Alerts (element 1164 of FIG. 28) and Powertrain and         Controls Manager (element 1142 of FIG. 28) functions,         operations, or processes.

FIG. 31 is a flowchart or flow diagram illustrating certain of the inputs, functions, and outputs of a Flight Path Optimization Platform (the FPOP) that may be used to determine or revise a flight path for an electric-hybrid aircraft that may be used as part of the inventive air transportation system. Each or a combination of the functions, operations, or processes performed by or under the control of the elements or modules shown in the figure may be performed by the execution of a set of instructions by a properly programmed processing element (such as a controller, state system, microcontroller, CPU, microprocessor, etc.).

An implementation of the Flight Path Optimization Platform may be used to determine the optimal flight path(s) for a hybrid-electric aircraft. This includes defining speeds and altitudes, and an Energy plan for each of a series of flight segments while satisfying the performance and cost objectives defined by the Flight Mode. The FPOP determines optimal paths across one or multiple flight tracks; in doing so, it takes into account aircraft and powertrain characteristics, weather conditions, ATC restrictions, hazards, etc.

Note that flight planning for a regional hybrid-electric aircraft with multiple power sources requires a more complex set of trade-offs than a conventional aircraft being piloted over long ranges. For instance, for a hybrid-electric aircraft the optimal flight altitude is determined by a combination of speed versus efficiency objective, flight distance, weather aloft, aircraft aerodynamics, available energy and power, and relative stored energy versus generator usage. In contrast, the designated flight altitude for a long distance commercial passenger or cargo flight may be set by one or more of FAA requirements, government policies, and coarse aircraft characteristics. This is a much simpler manner of determining a segment (or segments) of a conventional long range flight. Because of the complexity of the flight planning process required for the inventive aircraft and regional air transportation system, the FPOP is used to execute the required optimization processes both pre-flight and during flight (as conditions change) to determine an optimal flight path.

In addition to primary flight path planning, FPOP may also be utilized on a periodic basis in flight to update the flight path to the destination (given changes in winds, ATC routing etc.), and provide alternate destinations or flight paths in case of failures within the powertrain, or other inflight emergencies:

-   -   During flight, periodically identifies all airports within range         of the aircraft given current energy state. Results may be         displayed to the pilot in any manner of formats including a         range ring on a map, an airport highlight on a map, a simple         text list etc.;     -   In case of any emergency situation, immediately provides the         flight path to the nearest acceptable alternate airport; and     -   In case of a partial failure in the powertrain, FPOP will         identify alternate destinations available with the degraded         condition of the powertrain. Examples of partial failures would         include failure one or more energy storage units, generation         modules, propulsion motors etc.

In some embodiments or implementations of the FPOP platform or data processing system, an optimization process may be performed on two levels:

-   -   Level 1: A simple rule-based calculation using standard         libraries to set altitudes and speeds based on Flight mode and         distance; and     -   Level 2: Optimization across a range of viable altitude and         speed alternatives, building on Level 1 output.

In some embodiments, the FPOP platform may include or be configured to access one or more of the following functions, operations, or processes:

-   -   Path generation (element or process 1404 in FIG. 31). This         defines the Level 1 Flight path for each Flight track. This         module Constructs a 3D Flight path, defined by waypoints         (latitude, longitude, and altitude) connected by flight segments         (for example, cruise, climb, descent), with a speed and Energy         plan assigned to each segment. Cruise may consist of one or more         segments as required by altitude constraints and ATC routing. In         the case of Level 2 optimization, builds alternate Flight paths         for each track using the Path alternatives generation rules         module (1413). For each Flight path, determines target speeds by         leg using the Speed rules library, and determines the Weather         and Caution indexes by interpolating across provided conditions.         In general, the path generation process or element uutilizes a         core sequence which draws from libraries of performance         heuristics and airspace constraints, such as the following:         -   Path generation sequence first sets cruise altitude(s) and             flight speeds from heuristics. Climb and descent distances             are calculated to set intermediate waypoints, airspace             constraints are calculated as waypoints with altitude             constraints. Lastly, Power ratio (range extending generator             state) is set for each segment;         -   Path heuristics (element 1407 in FIG. 31) A library of             altitudes, speeds and energy source utilization based on a             database from extensive flight path optimization. For a             given range, weight and Flight mode, returns the optimal             climb and descent rates, cruise altitudes and speeds, and             range extending generator utilization times for a no-wind,             simple flight profile. Heuristics may be generated using the             process described herein;         -   Airspace constraints. (element 1410 in FIG. 31) Utilizes the             navigation database of airspace and terrain, combined with             the flight track, to determine minimum or maximum altitude             constraints, imposes the constraints along the Flight track             as start and end (latitude, longitude, and altitude)             waypoints;         -   Climb, descent library. (element 1409 in FIG. 31) A library             which returns time and distance to climb or descend between             two altitudes at the current weight, and speed provided by             the Path heuristics. Library may contain table look-ups with             interpolative smoothing, or compiled performance             calculations;     -   Environment Evaluation. (element or process 1412 in FIG. 31)         Determines whether environmental conditions warrant a level of         path optimization beyond the baseline heuristic path. Applies         weather and cautions data to the route of flight; any cautions,         significant wind, or significant changes in wind on the flight         path will require the L2 Path refinement.     -   Alternate flight paths generator (element or process 1413 in         FIG. 31). Builds a set of alternate Flight paths by altering         segment altitudes from the initial Flight path. Uses the         Breakpoint generator to further divide existing flight segments         with additional waypoints if warranted, and then generates the         set of all possible paths. The maximum number of paths (order         10) is limited by altitude constraints (minimum, maximum), and         airspace rules which require cruise at incremental altitudes         (eg: over the US, eastbound flights cruise at odd-thousand feet,         9000, 13000 etc.);     -   Break point generator (element or process 1414 in FIG. 31). This         routine compares the existing cruise segments against the         Cautions and Weather index data (elements 1403 and 1405).         Typically, waypoints are inserted at the boundaries of any         caution, and are also inserted at any location with significant         wind velocity change. Each additional waypoint increases the         degrees of freedom available to the optimizer;     -   Flight path optimizer (element or process 1408 in FIG. 31). This         varies the cruise speed(s) and Power ratio over a Flight path to         minimize an objective function while meeting constraints.         Optimization is performed for the current aircraft state, which         may include weight, stored energy, and range extending generator         fuel available, and environmental conditions which may include         winds, precipitation, temperatures etc. Results of the         optimization process may include the optimized Flight path, Air         path, Energy plan, and value of the objective function. The         flight path optimizer may include one or more of the following         processes, operations, functions, elements, etc.:         -   Objective function. Set by Flight mode, the Objective             function will affect the cruise speed and energy utilization             strategy. Example objective functions which “bracket” the             performance space would typically be maximum speed (minimum             time), or minimum energy. A more comprehensive objective             function may include one or several of the terms below, with             parameters defined by the operator: Objective function=Cost             of fuel+Cost of stored energy+Cost of engine maintenance and             reserves (amortized)+Cost of battery packs (amortized)+Cost             of passenger and crew time+Cost of aircraft+Cost of             emissions;         -   Optimization variables. Optimizer varies cruise speed(s) and             Power ratio. For example, a maximum speed optimization would             result in a high level of range extending generator usage,             while a minimum energy optimization would result in a level             of range extending generator usage dependent on flight             range, sufficiently short flights would not use range             extending generators at all;         -   Optimization constraints. May include one or several             constraints on the powertrain for performance and safety             including maximum discharge rate of stored energy, minimum             state of charge at any point during the flight, minimum             energy reserves at end of flight (note that optimization             occurs on a fixed Flight path; any altitude constraints have             been satisfied by the Flight path generator);         -   Optimization process. The optimization space is nonlinear             and may contain discontinuities with power generator state,             both of which preclude a closed-form solution. Optimization             requires modelling the aircraft and powertrain performance             over the defined Flight path in a time dependent manner,             using the current aircraft configuration and in the expected             flight environment. Performance models result in time             integrated totals which are used to calculate the objective             function (for example: fuel and stored energy consumed);         -   Flight modelling may be accomplished with dynamic             programming, for example a flight simulation including             representative aircraft and powertrain models (described in             greater detail below), or by simplified methods, such as             reduced order models (so long as discontinuous, time             integrated properties are correctly modelled). The method             described herein utilizes a flight simulator process with             representative aircraft and powertrain models, and             incorporates the impact of the current operating             environment;         -   Optimization algorithms. A flight profile, with a single             cruise speed variable may be optimized with a gradient             descent, or Newton's method. A multi-segment flight with             discontinuities in Power ratio may require more advanced             nonlinear algorithms, such as NPSOL;     -   Constraints Check (element or process 1406 of FIG. 31) checks         the results of the optimized flight path against required end of         flight energy reserves or other constraints which may be used to         shape the optimization space. Any path which could not be         optimized to meet constraints is discarded at this point as         non-viable;     -   Path evaluation (element or process 1402 of FIG. 31). This sorts         all valid Flight paths by objective function, returns the         default Flight path (as initially requested), and the optimized         (i.e., that having the minimum objective function) Flight path         with all relevant information;     -   Flight simulation models and modules. The flight simulation         model may be utilized by the Flight path optimizer (element or         process 1408 of FIG. 31) and is a flight test calibrated, single         degree of freedom, physics based performance simulation model         which calculates the expected power required for the current         phase of flight given the aircraft weight, velocity, air         temperature, and pressure, and rate of climb or descent. The         model utilizes Aircraft and Powertrain models in time stepping         and empirical routines to continuously calculate performance         along the Flight path in the presence of forecast weather. The         results are integrated totals for time, distance, and energy.         The integrated distance totals are the Air path. These models         may include one or more of the following:         -   Flight modules. Performance is calculated for each flight             segment by the corresponding flight module (e.g., takeoff,             climb, cruise, descent, landing, etc.). The modules are             initiated with aircraft and segment information, and return             the integrated performance for the complete flight segment.             A Table provides the details on inputs and outputs for each             module, where the modules utilize the Aircraft and             Powertrain models to calculate aircraft performance;         -   Aircraft model. The model may be initiated with the aircraft             state (altitude, velocity, power level, weight, turn rate             etc.), and operating environment (altitude, air temperature,             pressure, density) and returns the corresponding             instantaneous performance (energy usage, fuel burn,             acceleration, climb or descent rate etc.). The model may             utilize a combination of force and moment equations (C_(L),             C_(D), C_(Di), CM, F, and N_(Z)W), and comprehensive table             look-ups to determine instantaneous aircraft performance,             where:             -   C_(L)—Standard calculation of C_(L)=N_(Z)W/q/S;             -   C_(D)—Drag build-up through a Reynolds based skin                 friction methodology with form drag factors, corrected                 to test drag. Additional factors for cooling drag, flap                 and landing gear drags (if needed), excrescence and                 interference drags;             -   C_(Di)—Induced drag with baseline C_(L) ²/(π AR e), e                 provided by table look-up, function of C_(L), flap                 setting. Trim drag added as lift increment to main wing,                 and C_(Di) from the HT;             -   CM—Pitching moment, from aircraft weight, CG, and                 neutral point (speed dependent, from table look-up);             -   F—Thrust, both available and required. Powertrain and                 propulsor models called to determine SHP available,                 converted to thrust with aerodynamic propulsor model                 (table look-up of efficiency Vs. Speed). Thrust required                 calculated to balance drag (in case of non-maximum                 thrust), and used for energy/fuel burn; and             -   N_(Z)W—Load factor, expressed in fractions of (g), due                 to accelerated flight (turning, or pitch rate);         -   Powertrain model. Represents the physics of hybrid-electric             powertrain modules and propulsors. In response to a thrust             requirement from the aircraft, powertrain module distributes             power between range extending generators and energy storage             units, returns thrust available, range extending generator             state(s), fuel burn rate, and storage discharge rate. The             aircraft simulation tracks range extending generator run             time, total fuel burned, and kWh of storage used. Additional             information on the powertrain models is provided in the             powertrain section (e.g., elements 1153 and 1154 of FIG.             28).

As noted, the FPOP flowchart or flow control diagram shows the sequence of steps in an exemplary implementation of the FPOP process. These typically include:

1. The FPOP is initialized from the POCS (1409); data needed for the flight path generation/optimization may be gathered from several sources:

-   -   a. Pilot input Flight mode information (1410) is provided by the         POCS, which also includes energy state requirements (e.g., level         of reserve needed on landing)     -   b. The GPS/FMS provides the initial, pilot-input flight track to         be optimized (1412)—there may be more than one flight track         option (e.g.: multiple routes around, or over, terrain);     -   c. Weather information (1414) is uploaded over data link (ADS-B         in);     -   d. The aircraft data bus provides current operating or         environmental parameters (1416) including temperature, air         pressure, and if this is an in-flight update, aircraft position         and speed.         2. Data pre-processing (1420) converts the wide-area weather         information into a Weather index (1403) of interpolated weather         at the locations along the flight track (based on latitude,         longitude, and available altitudes). Similarly, sources of         caution (e.g., icing or precipitation) are pre-processed to         check for their possible effect on the intended route of flight;         data are provided in the Cautions index (1405).         3. The FPOP platform is called with the fully assembled set of         input data (step or stage 1401).         4. The Flight path generator (1404) creates a three dimensional         flight path from the provided 2D Flight track. The generated         path is defined by a set of waypoints (defined by latitude,         longitude, and altitude), that are connected by segments (climb,         cruise, descent) with speeds specified for each segment:     -   a. A library of path/performance heuristics (1407) provides         optimal climb rates, cruise altitudes, and descent rates.         Heuristics are corrected for aircraft current weight and energy         state;     -   b. The climb and descent library (1409) uses the rates (from the         heuristics) to provide climb and descent distances, which         determine intermediate waypoint locations;     -   c. Intermediate waypoints may be added to match airspace         constraints (1410), including constraints due to terrain; and     -   d. Waypoints are connected with flight segments; speeds and         range extending generator state are assigned to all segments         from the heuristics.         5. Environment evaluation (1412) examines the Flight path in         combination with the Weather and Hazards data to determine if         the path would benefit from optimization under real world         conditions:     -   a. If no additional optimization is necessary or would be         beneficial, then the path is provided to the Flight path         optimizer (1408);     -   b. If further optimization has potential benefits, then the         Alternate flight paths generator (1413) is invoked;         -   i. The Breakpoint generator (1414) may add additional             intermediate waypoints to the cruise segments based on             sources of cautions and/or winds aloft; this provides more             degrees of freedom in the optimization space;         -   ii. The altitude of each cruise segment is varied up to the             maximum (set by performance limits) and down to the minimum             (set by constraints). Speeds and energy source utilization             are again set by using the heuristics;         -   iii. The full set of possible paths is provided to the             flight path optimizer (1408).             6. Flight path optimizer (1408) varies the cruise speed and             Power ratio over the flight path to minimize an objective             function within any specified constraints. For each Flight             path, the optimizer generates an Air path, Energy plan, and             objective function. Note that a path may be discarded if no             viable Energy plan can be found.             7. All viable paths are sorted by objective function, and             the optimum path is identified and returned. Final outputs             (1430) are the Flight and Air paths, Energy plan, Required,             Reserve, and Arrival energies, and a value of the specified             objective function.

Flight Simulation Modules (note that these represent examples of a possible implementation) Module Inputs Procedure Outputs ALL Aircraft weight, CG, starting, ending altitudes, atmospheric conditions (pressure, temperature), wind velocity Takeoff Flap angle, runway Calculate speeds for VCLmax, VLO, Take off speeds, distances for information (slope, V1, V2, calculate acceleration distances, ground roll, obstacle clearance, surface type) times, and balanced field requirement normal, engine out, balanced field, per FAA regulations, terminates on initial climb rates and gradients, obstacle clearance normal, engine out, take off time, energy used, and generator run time Climb Climb type (energy/ Calculates acceleration to climb speed, Initial, climb rate, total time, fixed speed) then executes a time stepping iterative energy, fuel, and distance to climb, Speed (KIAS) calculation of either a fixed speed climb, final altitude, final speed, generator Climb power or maximum specific energy climb (Ps), run time with climb rate determined by excess power available. Terminates at final altitude. Integrates energy, time and distance for segment totals Cruise Speed or Mach, Calculates acceleration to cruise speed, Total time, distance, energy, fuel Termination criteria: followed by steady state cruise until burn, and generator run time range, or final meeting termination criteria. Single waypoint, or minimum DOF, iterative time stepping calculation weight/energy, of flight physics with a Shuts down the energy reserve range extending generator once required at end sufficient stored energy remains to complete flight with required reserves. Descent Descent rate (ft/sec) Calculates either a closed form solution Total time, distance, energy, fuel Final speed for a “best glide” descent (L/D max) at burn, and generator run time zero net thrust, or a time stepping, iterative descent with fixed power, and linear deceleration from initial to final airspeed. Terminates on final altitude Landing Flap angle, runway Calculate landing speeds, VCLmax, Approach, round-out, flare, and information (slope and Vref, V_(TD). Calculate four distances; braking (ground roll) distances. surface type), approach initial descent from obstacle clearance Time, energy, and fuel used. angle, obstacle height, to start of round-out, round-out, flare, generator run time. braking μ, residual and ground roll. Round-out uses an N_(Z) thrust value to determine vertical, and horizontal round-out distance.

Note that the flight path optimization (such as that performed by the FPOP, and as described herein) depends on parameters which affect aircraft efficiency and cost; these vary significantly between conventional and hybrid platforms as shown in the table below.

Parameter Hybrid-Electric Conventional Cost of fuel Cost of fuel + battery depreciation + charging Linear function of fuel consumed energy Total flight energy Fuel (kWh equivalent) + battery charging Fuel consumed (kWh equivalent) energy (kWh) Propulsion energy Range extending generator efficiency may Engine efficiency coupled tightly with propulsor efficiency depend on altitude, speed, or may be efficiency independent of both. Combination are function of speed, altitude Stored energy efficiency may depend on state of charge, and discharge rate Propulsor efficiency function primarily of speed Cost of engine Linear function of run time (constant power Varies from simple function of operating time maintenance output, and not equal to flight time) (same as flight time), to comprehensive function of number of power cycles and time weighted by hot section temperature. Optimum cruise speed Function of two distinct operating parameters: Function of Velocity and/or Mach, and weight. for a fixed altitude Aerodynamic and propulsor efficiency Total energy required, P * t (where t = d/V) and total energy available (P_(gen) * t + E_(storage)) The first is a function of weight and speed and easily calculable for any given weight. The second however is specific to the exact energy state and range requirement of the current flight. For any given weight, and altitude, optimum For a given weight, and altitude, there is a single, speed changes with energy state and range. minimum energy cruise speed Optimal cruise altitude Function of range, stored energy, flight mode, Highest altitude which can be reached given the and range extending generator power with range altitude

In some embodiments, an optimization process may be conducted in order to generate path or other heuristics for the FPOP Flight path generator, as described herein. Below is a table containing information regarding differences in the optimization process between that which might be performed for the inventive hybrid-electric regional air transportation system and that which might be used for a conventional aircraft and transportation system.

Optimization Process Hybrid-Electric Conventional Objective Objective function may be based around the “cost Objective function is based around the “cost function function” C_(E)/C_(T) (cost of energy/cost of time) function” typically C_(F)/C_(T) (cost of fuel/cost of Cost of Energy includes fuel burned in range extending time) generators, battery charging energy and battery “Cost of Time” includes all non-fuel costs depreciation. including maintenance (engine, airframe), crew, “Cost of Time” includes all non-fuel costs including depreciation (airframe), and any other costs maintenance (engine, airframe), crew, depreciation directly related to flight time (e.g: insurance). (airframe), and any other costs directly related to flight time (e.g: insurance). May also include externalities such as emissions costs Altitude Vs. Efficiency weak function of altitude. Efficiency strong function of altitude (increases efficiency in Typical altitudes 6,000-25,000 ft with altitude). optimization Optimizer selecting altitude for speed, not efficiency. Typical operating altitudes from 25,000 to 40,000 Climb speeds default to higher values (shallower ft (turboprop, regional jet). climb) regardless of Flight mode (C_(F)/C_(T)) to improve Over regional ranges, optimizer is forced to trade average speed w/out efficiency penalty; efficiency (lower fuel burn) for speed depending Optimizer balances cost of time with efficiency of on value of C_(F)/C_(T) stored energy utilization (which decreases at high A high C_(F)/C_(T) minimizes fuel burn, results in a discharge rates), and any sensitivity of range extending steeper climb at a slower speed to the highest generator efficiency to altitude. altitude possible for the given range to minimize Maximum altitude limited either by range, range fuel burn, and a flight idle descent to maximize extending generator power available, or aircraft ceiling time spent at cruise altitude. Optimizing for minimum energy will result in a flight A low value for C_(F)/C_(T) minimizes total time, at speeds close to aerodynamic best Lift/Drag, with results in a higher climb speed, lower climb minimum possible range extending generator usage. angle, lower cruise altitude, and earlier, powered Optimizing for maximum speed will result in a low descent to maximize average speed. angle climb to the maximum altitude at which full generation is available, with continuous range extending generator usage from takeoff through end of cruise. Calculation For a given altitude, weight, and range, best efficiency For a given altitude and weight, best efficiency of best speeds results in full depletion of the stored energy sources by cruise speed is an easily calculated, single value the end of cruise. minimum between aerodynamic and engine Increasing C_(T) increases range extending generator efficiencies usage time up to the full cruise segment (maximum Aircraft total energy is a continuous function of cruise speed) aircraft state (potential + kinetic) and fuel burned. For a given altitude, cruise speed is a function This combination of first order, continuous aerodynamic efficiency, energy and power available, dependencies allow optimization for best cruise and objective function, altitude with a range constraint with standard Range extending generator power is typically constant optimization techniques, such as energy up to a limit altitude, and then decreases; this may minimization. Cruise altitude then determines reduce the altitude for maximum speed in cruise best cruise speed and total energy required (fuel This combination of weak altitude sensitivity to burn). efficiency, multiple energy sources of varying properties and utilization rates, and discontinuous range extending generator properties with respect to time and/or altitude results in highly coupled, discontinuous optimization space which is not compatible with closed form integration or linear optimization solvers, requiring instead a nonlinear method (e.g: Neldor-Mead) coupled with flight simulation This level of optimization would be prohibitive as a standard flight function Instead, heuristics are generated prior to flight, which determines an optimal path and Energy plan starting point for no-wind operations.

FIG. 30 is a diagram illustrating an example flight path optimization for an aircraft that may be generated by the Flight Path Optimization Platform (FPOP) and used at least in part to control the operation of the aircraft in an embodiment of the inventive regional air transportation system. As shown in the figure, a flight path 1300 may be composed of one or more segments (such as those identified by “A”, “A.1”, “B”, “C”, “D”, etc. in the figure), where each segment may require a specific configuration of the aircraft and control systems in order to be properly implemented (e.g., subject to the constraints placed on the operation of the aircraft by travel distance, fuel (energy) level, fuel consumption, total weight, etc.). The figure shows a graphical example of the flight path optimization process in cross section, and thus only an altitude profile as a function of distance. In this example, the default flight path 1300 is a single origin, single destination path, which is broken into multiple segments by the Path Generation module/function of the FPOP.

The initial path (represented by the dashed line) produced by the Path Generation process of the FPOP module is based on the origin (A), destination (D), and the altitude constraint for the terrain obstacle. This default path results in an initial climb (the segment A to A*), a cruise mode (the segment A* to B) at an optimal no-wind altitude, a segment at higher altitude to clear the obstacle (B to B.1), a return to optimal cruise altitude when the obstacle constraint is removed (the segment B.1 to C) and cruise until the top of the descent point (the segment C to C.1), followed by the descent to landing (the segment C.1 to D). The path generation process uses the climb and descent rates to determine the intermediate points of the flight path (i.e., A.1, B.1, and C.1). Note that optimal climb and descent rates, cruise altitudes and speeds, and the generator off point (indicated by the triangle between points C and C.1) are determined by the Flight Mode and range. For example, a “high speed” mode over a medium range results in a best rate climb to the maximum altitude which allows peak generation power, with range extending generators on for all cruise, whereas an economy mode, over the same distance, may cruise more slowly, at a lower altitude, and range extending generator shuts down partway through cruise, completing the flight on stored energy alone. This path is provided to the Energy Optimization module, and then to the Path Evaluation module of the FPOP.

Returning to the example optimization process illustrated in FIG. 30, in some embodiments (and as suggested by FIG. 31), in a typical optimization process, an Environment Evaluation module/function 1412 checks the Weather Index 1403 and Caution Index 1405 for the potential cruise segments and determines whether further optimization should be performed based on the wind velocities, changes in wind direction or speed, etc. (as indicated by the “Yes” or “No” branch of the “Refine Path?” decision step 1415 of FIG. 31):

-   -   Breakpoint Generator 1414 first determines whether the existing         cruise legs (i.e., A, B and C), need additional subdivision         based on wind gradients; in this case, the answer is no, as the         wind is consistent on each leg (as suggested by the wind speeds         W1 and W2 shown in FIG. 30);     -   Alternate flight paths generator module 1413 varies the         altitudes at A.1, B, and C, which modifies the locations of         points A.3, B.1, C and C.1 of the flight path; note that there         are a limited number of feasible variations because aircraft         regulations require cruise to occur at incremental altitudes         (e.g., every 2,000 ft in the US). The lower bound for a cruising         altitude is set by the minimum route altitude (the MEA, defined         by the terrain and airspace), while the upper bound is set by         aircraft performance capabilities. The result of this variation         process is a set of potential flight paths;     -   Each potential flight path is analyzed by the Flight Path         Optimization module 1408 by implementing a flight simulation         process to find the lowest energy usage for that path; and     -   Path Evaluation module 1402 is used to rank the paths and return         both the default path(s) and the path(s) which minimized the         objective function.

In this example (as compared to the default flight path 1300 shown in FIG. 30), Path Optimization module 1408 decreases the initial altitude to the lower limit to avoid the head wind, moving back the location of A.3 to ensure sufficient distance to climb to B for clearance of the terrain obstacle. The altitude at B doesn't change, but after the obstacle is cleared, a lower cruise altitude at C takes advantage of a tailwind, and the top of descent point (C.1) is delayed as long as possible to utilize the tailwind. The reduced energy usage on the initial segment allows the generator to be shut down earlier (as suggested by the triangle nearer to point C in the figure).

The Table below shows each waypoint in the optimized path, the source of the intermediate waypoints, the desired altitudes and speed(s) for each leg, and how the optimization process modified the original default flight path. In the table, the speed and/or altitude of the A.2, B, B.1, and C waypoints have been optimized. The table also lists how the speeds are determined for each leg; note that legs which have been optimized for altitude have also been optimized for speed.

ID Name Source Speed Constraint Altitude A Departure Flight Track n/a Fixed Fixed point A.1 Top of Calculated Calculated Minimum A.2 initial by rule climb A.2 First Calculated Optimized Minimum Optimized to cruise leg for Flight lower altitude mode to avoid the strong headwind (W1) A.3 Start of Calculated Calculated Minimum A.2 second from by rule climb constraint at (B) B Cruise Flight Track Optimized Minimum Constrained by waypoint for Flight minimum mode altitude required to clear terrain B.1 Cruise Flight Track (B) Minimum (B) waypoint C Start 3^(rd) Calculated Optimized Minimum Optimized to cruise leg from descent for Flight altitude that distance mode delivers best combination of tail wind (W2) and range extending generator efficiency. C.1 Start of Calculated Calculated Minimum (C) descent from based by rule for on zero net- zero-thrust thrust (D) descent D Arrival Flight Track n/a Fixed Fixed point

As noted, flight path planning for a regional hybrid-electric aircraft with multiple power sources requires more complex trade-offs than a conventional aircraft over long ranges. For instance, optimal flight altitude is determined by a combination of speed versus efficiency objective, flight distance, weather aloft, aircraft aerodynamics, available energy and relative energy storage versus range extending generator or alternate power source usage. The FPOP processes enable this optimization both pre-flight and during flight as conditions change, to determine the optimal flight path or paths.

As described herein, in some embodiments the FPOP platform or system for a hybrid-electric aircraft may have the following characteristics and/or perform the indicated functions:

-   -   Generates one or more Flight path(s), optimized for the Flight         mode, and which meet both aircraft and environmental constraints         (e.g., final energy state, and airspace limitations). A         determined or revised Flight path may be uploaded to the FMS         (shown in FIGS. 20 and 21) to be executed by a pilot or         autopilot;     -   Flight planning for a regional hybrid-electric aircraft with         multiple energy sources requires more complex trade-offs than a         conventional aircraft, and is also more critical to flight         safety due to the complexity of using multi-source energy         reserves. Aircraft performance over a mission is inherently         nonlinear and typically approached with some level of dynamic         programming (e.g. simulation), coupled with an optimization         method, for example an energy method to minimize total flight         energy. Hybrid-electric performance entails additional levels of         complexity in that energy contributions are from multiple         sources, which are physically distinct (either an energy source,         or a power source), which may not be continuous with respect to         time, or cost over the flight. This results in a complex         optimization process not addressed by conventional flight path         planning;     -   As described, the Flight Path Optimization Platform (FPOP) for         hybrid-electric powertrains typically uses a two-step         process; (1) Flight path definition to set the overall flight         profile including cruise altitudes, followed by (2) optimization         in the current operating environment.         -   Flight path definition may occur on one or two levels             depending on the environmental conditions:             -   Level 1: Initial definition of a 3D Flight path using                 heuristics which provide the optimal altitudes, cruise                 speeds, and Energy plan for the desired range and Flight                 mode. If the flight environment is relatively simple                 (low winds, no cautions or hazards), then Level 1 is                 typically sufficient;             -   Level 2: Invoked to generate alternate paths when wind                 or cautions adversely affect the Level 1 path. Varies                 the altitude(s) of the Level 1 Flight path cruise                 segment(s) to generate a set of modified paths.         -   An optimization is performed on each Flight path by             adjusting cruise speeds and the Energy plan (Power ratio) to             minimize an objective function within constraints, while             taking into account the current operating environment             (weather, and aircraft state). Results are the Air path,             Energy plan, and value of the objective function. In the             case of multiple Flight paths, the path with the minimum             objective function is returned as optimum.

Compared to conventional aircraft operating on long haul flights, the regional, hybrid-electric aircraft flight profile has many more options for speeds and altitudes, and is significantly more complicated due to the use of multiple energy sources which respond differently to altitude and power demands, and have different costs. As part of this innovation the inventors recognized that conventional aircraft flight planning is inadequate to provide safe, efficient flight paths for hybrid-electric aircraft, and that this capability must be provided to ensure flight safety and reduce pilot workload. The implementation of the inventive FPOP platform/system is based on the recognition by the inventors of the differences between operating and optimizing hybrid-electric powertrains and those of conventional aircraft. These differences or distinguishing characteristics include:

-   -   Conventional, long haul aircraft use a limited set of prescribed         climb and descent profiles and cruise at altitudes between         31,000 and 40,000 ft. Cruise altitude is easily determined from         winds aloft and air traffic requirements, and “optimization” is         generally little more than an adjustment to speeds to adjust for         the price of fuel;         -   Regional aircraft spend a much higher fraction of the flight             path/time in climb and descent, and cruise altitudes vary             widely depending on range, weather, terrain, and air traffic             control. Even so, for conventional aircraft in regional             operations, the best cruise efficiency typically relies on             climbing to the highest altitude practical given the cruise             range;         -   Energy planning in conventional aircraft is typically the             process of ensuring that more fuel is available than is             required for the flight. Fuel burn is calculated from the             planned flight segments and the required reserves (expressed             as time, or time+diversion to alternate). Calculations are             made by the pilot or FMS using a system of table lookups             accounting for aircraft weight, cruise altitude and speed;         -   Cruise speed is chosen between a high speed cruise (maximum             power), or a long range cruise (best economy) depending on             cost of time and fuel available;         -   Conventional aircraft engines lose power with altitude, and             even a “full-power” (i.e. full throttle) flight will not             exhaust fuel reserves too quickly; and         -   Conventional flight path optimization is typically based on             a simple ratio of Fuel Cost to operating costs. For example,             some aircraft manufacturers call this the “cost index”, a             single number set by the operator, which the aircraft FMS             uses to set the climb speed, cruise speed, and top of             descent point.     -   In contrast, hybrid-electric regional aircraft are efficient         across a wide range of altitudes, have cruise speeds determined         by energy, not power available, and have a more complicated         total cost vs. energy cost:         -   Cruise altitude primarily affects speed, and power             source/range extending generator power available (which             affects range for a given speed). With much smaller changes             in efficiency; an optimizer will choose higher altitudes for             speed (minimize total cost), over energy efficiency (the             opposite of conventional flight planning).         -   Energy planning is significantly complicated by             dual/multiple energy sources with different operating             properties:             -   a. Stored energy provides a wide range of power,                 independent of altitude or speed, but with relatively                 limited amounts of energy. Stored energy may suffer                 efficiency losses as a function of power output,                 effectively reducing the amount of stored energy when                 operating at high discharge rates;             -   b. Range extending generator provides constant power,                 with total energy limited by fuel available. Range                 extending generator power and efficiency may change with                 altitude; and             -   c. Reserve energy must be specified for each source,                 sufficient to ensure that safe flight can be maintained                 at all times.         -   Cruise speeds range from maximum energy (total of stored             energy available for cruise+generation*time), which is a             function of range, and minimum energy; cruise speed is set             to match a target future energy state.         -   Electric propulsion does not lose power with altitude; a             pilot continuing to fly at maximum power at high altitude             will incur a much more rapid depletion of stored energy than             a conventional pilot would expect.         -   Flight path optimization trades total costs of energy and             power (stored energy costs+generation costs) against             operating costs. Differences in cost of stored vs. generated             energy extends the basic optimization to include energy             sourcing optimization (e.g., the POCS Hybrid energy planner             function).

As part of creating the inventive aircraft and regional transportation system, the inventors have developed a process or set of processes for the design and optimization of forward compatible hybrid-electric aircraft. The design process includes sizing of powertrain components, propulsion integration, wing sizing, and noise reduction which collectively result in an aircraft with 60-80% reductions in direct operating costs, 20-30% shorter runway capabilities, and 15-25 EPNdB lower noise than conventional aircraft. Moreover, forward compatibility ensures the aircraft can accommodate future EV/Hybrid technologies via relatively simple upgrades of specific powertrain modules. As a result, an embodiment of the inventive aircraft is expected to remain competitive over the target life of the airframe, offering improved performance and decreasing costs with module upgrades. In addition, the same or similar process can be used to develop aircraft variants with varying performance tailored to specific markets (via a choice of powertrain modules without any change to the external airframe or propulsors). This enables the development of aircraft optimized to specific markets with minimal engineering and re-certification requirements. This set of design and optimization concepts and processes for aircraft that may be utilized as part of the inventive regional air transport system will be described in further detail with reference to FIGS. 32 and 33. Note that conventional aircraft design processes are not able to size the hybrid-electric powertrain components, ensure the aircraft and powertrain stays abreast of rapidly improving EV/Hybrid technologies, or fully leverage the unique benefits of electric propulsion, including breakthrough efficiency, short take-off and landing capabilities and low noise operation.

FIG. 32 is a flow chart or flow diagram illustrating a hybrid-electric aircraft design process that may be used in implementing an embodiment of the inventive air transportation system. In some ways, the overall flow is similar to conventional aircraft design, but certain steps (those shown bolded) are either modified or entirely unique to the hybrid-electric design process. A table below provides a description of each of these changed steps, with a comparison to the conventional process.

The flow chart of FIG. 32 illustrates the primary components in the inventive aircraft design cycle. Aircraft design is a highly iterative process because of the interdependence of the key design parameters of weights (payload, fuel and aircraft), propulsion power, and key structural sizing (wing, empennage, landing gear, etc.). Note that the operations or processes shown bolded are those significantly impacted by the hybrid-electric powertrain and its use as part of the inventive regional air transportation system:

1. The design process starts with top level design requirements including payload, cabin size, the cruise speed and range, maximum altitude, the takeoff and landing runway performance, and, the noise and cost requirements (step or stage 1502); 2. Weight is the single most important driver in aircraft design. Maximum weight directly affects required engine power, wing size, and structural weight, as well as cruise power (energy) required. Each design cycle starts with an update to weights (step or stage 1504); 3. Based on maximum weight, the aircraft wing and tail areas are sized, and a rough performance analysis is used to determine required propulsion power, and energy capacity to meet range and speed requirements. The aircraft configuration is also laid out for location of major components, wing, tail, landing gear etc. (step or stage 1506);

-   -   a. Unique to the hybrid vehicle is powertrain component sizing,         taking into account the amount of stored energy and generation         power capacity (step or stage 1507). The 3-tier design process         described herein uses ranges and speeds, in combination with         cost, to provide the requirements and constraints needed for         this function;         4. From the sizing and configuration, weights can be built up         from a series of models and component weights (step or stage         1508). For example, wing parametric weights take into account         thickness, span, area, sweep, and taper to estimate typical         weights from the geometry, while engines and landing gear are         typically fixed, component weights provided by the supplier;         5. The sum of all aircraft weights results in the empty weight;         if empty+payload+fuel+stored energy weight exceeds the maximum         weight, then the relevant portions of the design process will be         performed again with updated weights;         6. Performance modeling (step or stage 1510) is now used to         estimate aircraft performance; this involves application of the         representative aerodynamic and propulsion models using the         weights, configuration, and powertrain information developed         from the sizing steps;         7. Note that hybrid-electric propulsion requires two independent         models; one for the propulsor, one for the powertrain (step or         stage 1511);     -   a. The propulsor model combines the motor sized in step (3),         with the aerodynamic characteristics of the propulsor (e.g., the         propeller) to calculate power required for a specified thrust         level; and     -   b. The propulsor model determines the thrust available based on         the powertrain model and includes the stored energy units and         generator. The powertrain model determines maximum power         available, and, for a given power requirement, the ratio of         storage to generation, storage discharge rate and range         extending generator fuel burn.         8. Lift and drag models are based on the geometry of the         configuration (step 3) and enable performance calculations in         various configurations, including cruise, take off, landing,         flaps up, and down, landing gear up, and down, speed brake         deployed, etc. Note that electric propulsors may be used with         regenerative braking to replace conventional spoilers;         9. Performance modeling employs physics based models which may         include numerical approximations, and time stepping methods to         calculate the time, fuel, energy, distance, and altitude change         for each step;     -   a. The performance models may be modified from conventional         versions to control and track both propulsor and powertrain         models. This includes controlling generator on/off, and tracking         fuel burn and run time, and stored energy usage; and     -   b. Costs may be calculated by applying the performance model to         a representative flight path, and applying cost values to the         integrated totals of times, fuel burn, and stored energy used.         10. Performance is now checked against design requirements;         deficiencies will require design changes, and another design         cycle (step or stage 1512).

The table below provides a description of certain of the changes to a conventional aircraft design process that were developed by the inventors for a hybrid-electric design process, along with a comparison to the conventional process.

Area Hybrid-Electric process Conventional process Design requirements Cruise range, Define a three tiered set of ranges and speeds: Define a maximum range requirement to and speed A: range and speed the aircraft can fly on stored energy be met at long range cruise speed. alone, range extending generator needed only to meet Define maximum cruise speed reserve requirements. B: Optimal speed hybrid range which fully depletes stored energy in combination with range extending generators. Also defines minimum cruise speed when flying to the full distance. C: extended range primarily using range extending generators and flown at lower speeds Define maximum cruise speed Forward Define a set of expected changes in powertrain components Not considered. compatibility over the aircraft lifespan. Includes improvements in energy storage, power generation, and propulsion motors. Three tier range and speed requirements re-defined at these future technology levels Net result is an “envelope” of design requirements covering current and future ranges and speeds. Cost Cost requirements specified in the 3-tier process used for Cost requirements based on evolutionary speeds and ranges including forward compatibility, for improvement over existing aircraft with example: small changes in several areas A: minimum cost, up to 80% reduction over conventional. Costs increase at shorter ranges. B: 60-70% lower DOC than conventional aircraft C: 30-60% lower DOC DOC reduction is maximized on shorter flights, the opposite of conventional Range Vs. High fixed mass for energy storage, and low fuel burn in The maximum sum of fuel weight and Payload hybrid mode results in minimal range payload trade; instead payload weight is a constant; this results these values were already captured: in two cruise range requirements: Range with maximum payload is maximum hybrid range Range with maximum payload (B) Absolute maximum range as limited by Absolute maximum range is the requirement for range (C). fuel volume Maximum Conventional propulsion limits do not apply since motor Maximum operating altitude limited by altitude power available is not affected by altitude. thrust lapse in conventional aircraft Other physics limits may apply; for example voltage engines breakdown (corona) limits maximum voltage as a function of altitude Barring physical limit, the designer must pick a rational ceiling based on intended cruise altitudes, and level of pressurization. Runway The selection of length is same as conventional. However, Minimum runway length is selected to be lengths motor peak power capabilities substantially alter the takeoff the longest runway which still meets the design process allowing a shorter runway requirement than target market requirements. an equivalent conventional aircraft without compromise Weights Weight Same as conventional, however, for the payload fraction to Top level estimation uses a representative Estimation be representative, payload weight includes all passengers, payload mass fraction (PLF), typically cargo, fuel, and energy storage weight between 0.55 and 0.6 for regional aircraft W_total = W_Payload/(1-PLF) Payload weight includes all passengers, cargo, and fuel Initial fuel requirements are estimated from cruise range and average fuel burn per mile. Configuration and Sizing Wing sizing, Weighted multi-point optimization with constraints. Wing design optimized for cruise planform Operating points are: condition with constraints Hybrid cruise (multiple points for full range of time variant Constraints: takeoff and landing distances speeds) (incl. balanced field requirements) C Cruise (generation only speed) Constraints are: Takeoff and landing distances (incl. balanced field requirements) Continued flight on only energy storage or range extending generators alone. Vertical tail For a single propulsor, there is no change. For a multi-engine aircraft, vertical tail sizing For a multiple-propulsor aircraft, vertical tail sized to meet sized by engine out yaw requirement. directional control requirements during maximum motor Yaw moment depends on moment arm emergency power after propulsor failure; this may be more and drag coefficient of the failed engine in rigorous than a standard engine-out failure due to the very combination with thrust on the remaining high emergency power capability. The designer may need engine(s). to trade emergency peak power benefits vs. vertical tail sizing. Propulsion Propulsion motors are sized independently of the Engine(s) sized by the most demanding of power powertrain. Minimum power output determined by: three conditions: required Take off distance with maximum (routine) peak power Take off distance; and in case of a multi- For a multi-engine aircraft: balanced field length, with engine aircraft, balanced field length. benefit of emergency peak power. Top of climb thrust sufficient to accelerate Maximum cruise speed to cruise Minimum climb rate Maximum cruise speed Power for systems is delivered by the powertrain, and does The engines must be able to meet these not affect thrust power available. conditions while providing the additional power needed for aircraft systems. Powertrain Range extending generator capacity (kW), and stored N/A; fuel volume set by range component energy capacity (kWh) are sized as a system to meet the 3- requirements. sizing Tier range and speed requirements including forward compatibility envelope. Performance analysis over standard flight profiles is needed to determine mission energy requirements for stored and range extending generator sources. Minimum all-electric range (A) Criteria may set stored energy minimum, and must be specified in conjunction with some minimum level of kW/kg storage density. Minimum cruise speed to full hybrid range (B) Jointly sizes range extending generator power and stored energy capacity to meet the speed-range requirements across forward compatible envelope. Ratio of storage to generation will either be sized by constraints or cost optimization. Minimum cruise speed on range extending generators only (C) Provides a minimum sizing constraint on power generation. An additional safety constraint may also be applied as a requirement to be able to continue flight, including climbs, on either range extending generators, or energy storage units alone in case of partial system failure. In all cases, the powertrain must supply not only the required propulsion energy, but also the energy for all aircraft systems including ECS, flight controls, landing gear, avionics, etc. Propulsion High power density and high efficiency over a large range Maximum efficiency requires using the integration of power output levels allow the designer to potentially minimum number and maximum size utilize numerous smaller motors as easily as one or two engines possible. large ones. The monolithic nature of the engines results in an aircraft design and integration very tightly coupled to a specific engine with very few viable engine locations on the airframe. Powertrain The hybrid electric powertrain is distributed and modular Fuel system typically comprised of and energy which requires sufficient space for energy storage devices, multiple wing tanks, and related plumbing storage range extending generators, fuel tank(s), and all related to interconnect and cross feed tanks such integration power electronics. that any engine can draw from any fuel tank. Cooling Batteries, power electronics, motors, and generation all Engines and accessories are primary areas generate significant levels of heat, which must be removed. needing cooling, and are typically located Heat generating systems may be embedded in the fuselage in engine nacelles which have ready without ready access to cooling flow. access to cooling flow. These heat loads vary dramatically by power output levels from near zero to over 7% of power output. Cooling system should be designed to produce little or no drag across full range of thermal load Noise Electric motors, batteries inherently very quiet Noise reduction options primarily rest reduction Range extending generators integrated for noise reduction with propulsion provider (including with compartment insulation, muffling etc. propeller if turboprop). Electric motors ideal for integration into a ducted fan specifically designed for low noise operation Flexibility in propulsor integration allows more opportunities for noise reduction, by shielding propulsors over airframe structure, and/or keeping propulsor tip speeds low through gearing and/or motor design Performance Modeling Performance modeling assumes that a designer experienced in the art is using one or more performance methods to calculate aircraft performance in takeoff, climb, cruise, descent, landing, hold, etc. These methods may be empirical approximations, or may be time stepping integrations of the aircraft flight path. The following are changes required to a standard aircraft performance model to properly utilize, control, and track a hybrid-electric powertrain with electric propulsors. Propulsor Separate models for thrust generation and power generation Power and thrust generation typically and Motor power available is only a function of electrical power unified in a single “engine model” powertrain available (I V) and is independent of altitude and airspeed. Engine power is a function of altitude, models A simplified model may assume that the motor will put out airspeed, temperature, and usually full power when commanded, regardless of flight condition. provided in large tabulated decks or Thrust available is calculated from the combination of compiled subroutines motor power available and the aerodynamic model for the Systems powers is pulled from the propulsor (e.g. propeller or ducted fan). engines and usually included as part of the Range extending generator power available may be engine deck with no additional accounting function of altitude and/or speed. Fuel burn is function of needed. This includes overhead needed to range extending generators only. run the engines such as fuel pumps. Stored energy discharge rate calculated from power No additional losses are incurred due to required less generation power. fuel distribution Stored energy available is not a function of altitude or speed. Stored energy model may need to address effect of discharge rate on energy discharge efficiency Aircraft systems power is provided by the powertrain in addition to propulsion requirements. Total power required is then the sum of propulsion power, systems power, and losses in transmission which are implementation specific and may be modelled as a set of efficiencies between power sources and motors. Powertrain control includes range extending generator state (on/off/power level) either commanded or determined by rules of operation Air Brakes Regenerative braking provided by propulsors, no separate Drag is produced by spoilers surface used, no thrust produced during regeneration. (aerodynamic surface) which may be used Drag is a function of regeneration power extracted and regardless of engine thrust. aerodynamics of the propulsor in this mode. Drag is a function of spoiler deflection angle and Mach number Take off Standard takeoff calculation is modified to include peak For balanced field calculations on a power (standard takeoff), and the very rapid application of multi-engine aircraft, there is limited or no emergency peak power following a detected propulsor thrust increase available on remaining failure. engine(s) Climb Power commanded as % maximum (continuous), which Power commanded as % available doesn't change with altitude. Range extending generators (continuous), which is a function of speed, may be used for any fraction of climb from 0-100%, altitude and temperature. including shutdown of generator at a specified energy state. Cruise Speed selected to provide desired level of energy storage Speed selected cost and range depletion by end of cruise. Range extending generators requirements may be used for any fraction of climb from 0-100%, including shutdown of generator at a specified energy state. Descent Range extending generators off, propulsion thrust (and Engines always on with minimum fuel power) may be zero, or negative providing regeneration burn and thrust, even at idle power Flight totals In addition to standard time and distance integration, Track time, distance, and fuel burn hybrid-electric propulsion system requires tracking and Engine time not usually tracked separately- integration of additional time and energy quantities same as block time. Energy storage units: discharge rate, total discharge, and Total energy is the same as total fuel burn remaining capacity. Also track energy from regeneration or charging from generator. Range extending generators: fuel burn, run time, and power generated Total energy consumed (including systems and propulsion) Times to track include block time, flight time, range extending generator run time, and motor peak power application times.

Note that at least the following represent changes to a conventional aircraft design process that were developed by the inventors for a hybrid-electric design process:

-   -   Design requirements extended to enable sizing of key powertrain         components in a way that ensures compatibility with EV         technologies over the target life of the aircraft. This is         accomplished with the mentioned 3-tier set of ranges and speeds         for electric, hybrid, and extended cruise flight, specified         across a range of future EV technologies. An example of this         approach is shown in FIG. 34, with ranges and speeds for         regional operations and three levels of powertrain technology,         representing forecast performance 15-20 years into the future;     -   In contrast, conventional design requirements are typically for         maximum speed and range targets, using a specific engine which         will remain fixed for the life of the aircraft;     -   Wing design conditions and constraints extended to match the         3-tier ranges and speeds;     -   Wing design is a weighted multi-point optimization to account         for variation across the 3-tier set of ranges and speeds, with         maximum cruise efficiency at the optimal hybrid speed, and very         good efficiency for climb, electric only, and extended range         cruise speeds. Conventional wing design is typically focused on         a narrowly defined long-range cruise condition;     -   Take-off performance is typically the constraint for minimum         wing size, and this is somewhat mitigated by high peak power         capacity from electric propulsion motors. Peak power may be         applied to the balanced field sizing requirement, restoring much         of the thrust lost after a propulsor failure, and dramatically         reducing the “engine out” distance to climb. This results in a         smaller, more efficient wing in cruise for a given runway         requirement, and is not available with conventional engines         which are limited to, at most, a 10% peak power increase for         emergencies;     -   An additional minimum wing size constraint unique to         hybrid-electric propulsion may be added to the design process,         which is to ensure that flight operations may be safely         continued after any one energy source failure, reducing         powertrain output capacity;     -   Propulsion system sizing includes both thrust generation         (propulsion motors) and hybrid-electric power generation (stored         energy and generation power), whereas conventional approaches         only size thrust generation;     -   Propulsion motors are typically sized by single-point         performance criteria, the common three are take-off distance,         top of climb performance, and maximum cruise speed. Electric         motors affect these sizing points;         -   Take-off power can use a peak power significantly above             maximum continuous for a limited period of time. This allows             a smaller motor to meet the same take-off requirement;         -   Motors don't lapse (lose thrust) with increasing altitude;             as a result, electric aircraft are rarely power limited at             top of climb or in cruise; and         -   This combination of features allows selection of a smaller             motor, but this may leave the aircraft with less sustained             climb rate than expected, leading to an additional sizing             point of minimum sustained rate of climb.     -   Hybrid powertrain output component sizing for stored energy and         generation power cannot be performed based on point performance         conditions. Instead, these are sized using performance modelling         over a set of mission profiles defined by the 3-tier range and         speed requirements, including future technology levels. Sizing         is determined by minimizing an objective function, within         constraints of system weight, volume, and minimum power         available from either source for safety;         -   The objective function may include one or several of the             terms below, with parameters defined by the operator:             Example Objective function=Cost of fuel+Cost of stored             energy+Cost of engine maintenance and reserves             (amortized)+Cost of battery packs (amortized)+Cost of             passenger and crew time+Cost of aircraft+Cost of emissions     -   Electric propulsion integration separates the propulsion power         available (motor) from the thrust producing propulsor (fan,         propeller). The designer sizes motor power levels with an         assumed efficiency, and then propulsors are designed to spec.         This functional separation is enabled by electric motors         operating at high efficiency regardless of size, and being         easily integrated with propellers, rotors, ducted fans etc. In         contrast, conventional propulsion engines are monolithic units         of power and thrust generation combined, and once chosen,         channel the aircraft design along the few viable paths for         integration (e.g., commercial jets always have engines under the         wings);     -   As an example, the embodiment shown in FIG. 33 features three         ducted fans for low noise and enhanced take-off performance,         with noise further reduced by shielding from fuselage and tail.         Enhanced drag reduction is accomplished through clean, laminar         wing, fuselage boundary layer ingestion, and shorter, lighter         fuselage with wake fill-in from the ducted fans;     -   Propulsion models used in performance modelling are enhanced for         the hybrid-electric design process to represent propulsion power         and thrust, power generation from multiple sources, system         efficiency losses, non-propulsive power being used, and the         ability to store energy from regenerative braking. In contrast,         conventional propulsion models are simpler, and typically         represent an engine by providing thrust and fuel burn for the         current flight condition;     -   Motor models used in the inventive system and methods provide         power consumption as a function of torque, rpm, and controller         losses. Models also represent motor capabilities for time         limited peak power outputs;     -   Power generation models used in the inventive system and methods         represent the properties of each source, and losses due to         transmission and conversion. For example:         -   Stored energy is not dependent on altitude or speed, and can             put out a wide range of power levels; however, high             discharge rates are inefficient, reducing the total energy             available, and peak power output decreases as the stored             energy level drops;         -   Generated energy consumes fuel to provide power at a fixed             level; in contrast, power and fuel efficiency are typically             altitude dependent in conventional models; and         -   Efficiency factors are specified for losses in power             electronics and wiring;     -   A propulsion model may include the availability of regenerative         braking, using the propulsors to recharge stored energy on         descent, including the losses from motor and controller         efficiency, power transmission and conversion, and stored energy         charging efficiency;     -   Performance modelling methods are enhanced to separately control         and track power output (and power generation). Conventional         performance methods control engine power, and track fuel burn.         With the hybrid-electric powertrain, the model controls motor         power, range extending generator state (on/off/power output),         stored energy power (charge or discharge), and tracks stored         energy used, fuel burn, and range extending generator run time         (which is different from flight time). These changes to the         methods are needed to analyze hybrid-electric aircraft         performance, and to use performance modelling for powertrain         component sizing and optimization; and     -   Performance modelling methods may be further enhanced to         incorporate rules of powertrain operation such as “stored energy         first”, and “generation-off during descents”.

The inventive range-optimized hybrid-electric aircraft which is designed for maximum efficiency in regional operation may incorporate one or more of the following features, techniques, aspects, or elements, which collectively enable a 60-80% lower DOC than conventional aircraft:

-   -   Capacity of the energy storage units and the output of the         range-extending generator are optimized for maximum efficiency         over regional ranges. This results in 60-80% lower DOC than         conventional aircraft, via stored energy units at 12-20% of         maximum weight of the aircraft, and range-extending generators         operating at less than 70% of maximum continuous output of the         powertrain. This is in contrast to less efficient or practical         designs for hybrid aircraft that are focused on longer ranges,         and yield <30% reductions in DOC over conventional, based on         lower stored energy capacities, and higher generator outputs;     -   Aircraft designed to minimize an objective function across the         3-tier requirements, primarily weighted for hybrid cruise         requirements (B);         -   the objective function may include one or several of the             following terms, with parameters defined by the operator:             Objective function=Cost of fuel+Cost of stored energy+Cost             of engine maintenance and reserves (amortized)+Cost of             battery packs (amortized)+Cost of passenger and crew             time+Cost of aircraft+Cost of emissions;     -   The previously mentioned 3-tier set of speed and range design         requirements is used for powertrain and aircraft design, an         example of which is shown in FIG. 34. In one embodiment, these         tiers are defined by:         -   Range A: Highest efficiency (80+% lower DOC than             conventional aircraft) and optimal speed over electric-only             range.         -   Range B: Intermediate efficiency (60-70% lower DOC than             conventional aircraft) and optimal speed over larger hybrid             range.         -   Range C: Good efficiency (30-60% lower DOC than conventional             aircraft) and lower speeds beyond to maximum range             determined by onboard stored energy and fuel less safety             reserves.     -   As an example of the inventive design process, the table below         compares fuel burn between a conventional turboprop and a range         optimized hybrid-electric for each phase of a regional flight.         Note that the hybrid fuel burn is 72% lower than the turboprop         for the full flight, with reductions of nearly 90% in take-off         and climb, 67% in cruise, and 88% in descent and landing;     -   Fuel burn (lbs) by flight segment, 345 mile range     -   The inventive aircraft is designed for efficient operation at         lower altitudes, with a target of a 50-90% lower fuel burn than         conventional aircraft. As noted, regional operations typically         involve a higher fraction of the flight time in climb or descent         and low altitude cruise compared to conventional long range         aircraft. This imposes a much greater emphasis on operational         efficiency during these phases. Accordingly, the inventive         hybrid-electric aircraft are designed for a 70-90% lower fuel         burn in climbs and descents than conventional aircraft, and a         50-80% lower fuel burn in cruise at altitudes of 4,000 to 30,000         ft. and speeds of 150 to 400 mph, than conventional aircraft. In         some embodiments, this is accomplished by one or more of the         following:         -   Propulsion by electric motors which deliver high efficiency             independent of altitude or speed, and do not consume energy             when not under load. In contrast, aircraft gas turbines             suffer from 30-50% lower efficiency at lower altitudes and             speeds, and require a minimal fuel burn even at flight idle;         -   The aircraft is designed to maximize flight on stored energy             units to extent possible, given the lower total cost             relative to the range-extending generators. This translates             into outstanding low altitude, low speed performance given             stored energy units, e.g., battery packs, which offer very             high conversion efficiencies that are independent of             altitude or speed. This is in contrast to aircraft engines             wherein fuel efficiency is highly altitude and speed             dependent;         -   The pairing of the propulsion motors with low pressure             variable-pitch propellers or ducted fans that are designed             for high efficiency across a range of low and intermediate             speeds (e.g., 150-300 mph) typical of regional operations.             In particular, these provide far greater efficiency in             climb, or low altitude cruise than a conventional turbofan;     -   The aircraft is designed for very high efficiency during near         airport operations, e.g., taxi, takeoff, approach, landing, with         a target of a >90% reduction in fuel burn compared to         conventional aircraft operating in these modes;         -   Taxi, approach and landing are designed to be electric-only,             utilizing the highly efficient stored energy units. Unlike             the necessary minimum fuel burn of aircraft gas turbines,             hybrid-electric aircraft consume no fuel in these phases             given that the generator is switched off;         -   Descents designed to be flown at zero energy, with             generators turned off, unlike conventional aircraft engines             which require sustaining fuel burn at idle;         -   Steeper descents are enabled by regenerative braking of the             electric propulsors, enabling energy recovery, unlike the             use of drag producing devices such as spoilers in             conventional aircraft; and         -   Take-off uses a combination of stored energy units and             generators, translating to much lower fuel burn than             conventional aircraft;     -   The aircraft is designed for quiet, short take-off and landing         (STOL) operations with 15-25 EPNdB lower noise of operation, and         requiring runways 20-30% shorter than conventional aircraft,         both with minimal impact on cruise efficiency;         -   The aircraft is designed for 15-25 EPNdB lower noise as             measured by standard CFR 14 part 36 criteria. This is             accomplished by design to limit and suppress noise             generation across the three primary sources of aircraft             noise; power generation, thrust generation, and airframe:             -   Power generation noise is reduced significantly given                 the use of electric propulsion motors, and energy                 storage units which produce no significant noise.                 Meanwhile, the range-extending generator is downsized to                 <70% of maximum continuous power and integrated in a                 noise insulated chamber within the airframe, e.g.,                 embedded in the rear fuselage;             -   Thrust generation noise is reduced significantly by the                 use of low-noise variable-pitch propulsors, such as low                 RPM, quiet propellers, or variable-pitch ducted fans. In                 addition, propulsors may be integrated in the airframe                 in ways that shield noise from propagating to the                 ground, e.g., using aircraft wings, fuselage, or                 empennage flight surfaces; and             -   Airframe noise is reduced significantly via regenerative                 braking with the low-noise propulsors instead of                 conventional spoilers;     -   Aircraft operations may be optimized for further noise reduction         by leveraging unique features of the hybrid-electric powertrain:         -   Quiet taxi, descent and landing on energy storage units,             with generator switched off;         -   Take-off noise reduced with a shorter ground roll and steep             angle climb-out over noise sensitive areas. This is enabled             by the high peak power capability of the electric propulsion             motors, and by use of a low-noise ducted fan for high static             thrust; and         -   Approach and landing noise are reduced by steep, controlled             descents with regenerative braking, using the variable pitch             electric propulsors;     -   The aircraft and associated flight operations are designed for         use with 20-30% shorter runways than conventional aircraft by         leveraging features of the hybrid-electric powertrain to         accomplish this without the typical performance penalties.         Similar STOL performance in conventional aircraft would require         larger wings and engines, resulting in reduced efficiency and         payload;     -   The design achieves a thrust boost during take-off by leveraging         peak output capability of the electric propulsion motors,         thereby enabling STOL operations without a need to upsize motors         (e.g., a 20% boost over continuous output for 2-4 minutes during         takeoff and initial climb);     -   The design achieves a shorter balanced field without larger         wings or engines. “Balanced field” calculates maximum runway         required following an engine failure during takeoff, and         balances the distance required to either stop on the runway, or         continue the takeoff on the remaining engines up to an obstacle         clearance height (FAA standards are 35 or 50 ft). Balanced field         (and hence required runway) is dominated by rate of climb on the         remaining engine(s); as part of the innovative system, climb         distance following a failure is dramatically reduced by boosting         surviving propulsors up to 200% in the event of a partial or         complete failure, and stopping distance is reduced by rapidly         dropping thrust to zero or negative (thrust reverse). Detection         of failures and the boosting of thrust to compensate are managed         automatically by the inventive powertrain optimization and         control system (POCS). Similar thrust over-boost systems in         conventional aircraft are limited to <10% boost, while stopping         distances are hindered by spool-down time and thrust residuals         on surviving engines;     -   In the event of a partial or complete failure of a propulsor,         e.g., due to a bird strike, or loss of one or more propulsor         motors in flight, the POCS boosts power to the surviving         propulsors to compensate for a limited period, thereby enabling         an extended reaction time window for the pilot to take         corrective action, and providing safe descent to a nearby         airport or landing area;     -   Unlike the limited boost capabilities of conventional aircraft         engines, electric propulsion motors can boost up to 200% of         continuous power for limited time periods, typically determined         by the system's thermal limits; and     -   Variable-pitch propulsors coupled with electric motors enable         very quick reduction of thrust to zero, translating to shorter         stopping distances than aircraft gas turbines given spool-down         time and thrust residual effects.

As mentioned herein, the inventive aircraft and design process are intended to provide forward compatibility across the airframe, powertrain and propulsion system(s). This is accomplished by incorporation of several underlying principles or design guidelines:

-   -   The aircraft is designed to accommodate upgrades to future EV         technologies over the life of the airframe, including improved         flight performance enabled by the upgrades. Given the rapid         evolution of EV technologies, this feature ensures the aircraft         remains competitive over time as technologies improve (e.g.,         batteries, supercapacitors, electric motors, internal combustion         engines, fuel cells, etc.). In addition, this feature enables         the aircraft to transition smoothly from hybrid-electric to         all-electric once energy storage technologies improve to the         point where range-extending generators are no longer required.         The ability to upgrade components of a hybrid-electric         powertrain for step-change performance improvement is unique to         the inventive hybrid-electric aircraft, and a contrast to         conventional aircraft which have largely monolithic engines;     -   To ensure forward compatibility, the inventive hybrid-electric         aircraft are designed at multiple points, with a powertrain         sized for speed and the 3-tier range requirements (A), (B)         and (C) noted, but are based on technologies available at         aircraft launch and forecast to be available over its target         life (including a potential transition from hybrid-electric to         all-electric for some designs). This leads to a forecast for the         onboard powertrain, and in turn, determines performance         characteristics over time, such as speeds, electric and         hybrid-ranges and operating costs (with the expectation of         electric-ranges increasing and operating costs decreasing as         technologies improve);         -   Aircraft are designed for multiple discrete powertrains,             reflecting forecast upgrades to improved EV technologies             over the target design. For instance, these could include             energy storage densities changing from 300 to 1,200 Wh/kg,             motor power densities from 4.5 to 10 kW/kg, and internal             combustion engine power densities from 1 to 5 kW/kg. The             aircraft design cycle is repeated for each of the discrete             powertrains, by adjusting the 3-tier range and speed             requirements for the progressively improving EV             technologies;         -   In the example shown in the table below, each row represents             a discrete powertrain based on EV technologies available at             a point in the future. For each discrete powertrain, the             speed and range design requirements (A), (B) and (C) may be             determined by minimizing an objective function, for example             (DOC+I+COT). These individual requirements define a lifetime             envelope of design points including speeds, ranges,             altitudes, that the aircraft must be designed for over its             target lifetime;

Range: B C Storage Motor ICE ICE A B min C min Density Density Density efficiency electric Hybrid speed Range speed (Wh/kg) (kW/kg) (kW/kg) (BSFC) (miles) (miles) (mph) (miles) (mph) 350 5 1.1 0.35 80 400 220 700 205 600 7 1.4 0.33 140 500 250 750 220 1000 8 1.4 0.33 235 550 270 850 225

-   -   -   The airframe and propulsor are designed to operate             efficiently across this lifetime flight envelope, typically             translating to faster and higher flight over time (as shown             in FIG. 34), as energy storage technologies improve; and         -   One outcome of the design process described herein is the             recognition that forward compatibility typically limits the             weight of the rechargeable energy storage units to 12-20% of             the aircraft weight, so that payload capacity is roughly             uniform as EV technologies improve. Higher weight fractions             would lead to aircraft that are larger and heavier than             aircraft of similar payload in the initial years, with             payload increasing over time, while lower fractions lead to             suboptimal efficiencies given much higher use of             range-extending generators.

As described, in some embodiments, the inventive hybrid-electric aircraft are designed to integrate with a modular hybrid-electric powertrain, including features to ensure the powertrain can accommodate a range of EV technologies by relatively simple replacement of compatible modules (such as rechargeable storage units, range-extending generators and electric motors). This may be accomplished by designing the airframe with bays that accommodate a range of current and forecast modules, along with access for module replacement. Compatible modules are those that are designed for operation with the powertrain platform, and that are supported by the design of the aircraft. These may include standard and extended energy storage units, high and low power range extending generators, and alternative energy storage technologies. Such features may include:

-   -   Multiple bays designed to accommodate rechargeable energy         storage units, standard or extended, not all of which may be         utilized on any particular flight, and some of which may be         multi-use space (e.g., generator, storage unit, fuel tank, or         cargo). Each bay provides structure, wiring, and access to         enable quick install or removal of the storage units. These         could include combinations of the following (some of which are         shown in FIG. 22):         -   Internal to the wing; standard and extended;         -   External to the wing in aerodynamic pods;         -   In mid fuselage, positioned under main cabin; and         -   In the rear fuselage, in addition to, or replacing             generators or cargo;     -   The modular energy storage bays may be integrated directly into         the primary aircraft structure, for example the wing spar box,         such that the modules serve a dual purpose of energy storage         containment and primary load path. Presence of energy storage         units within the modules may further enhance the strength of the         primary structure for increased structural efficiency and         reduced weight;     -   Energy storage units or other systems which require cooling may         utilize the aircraft skin for heat rejection. This cooling may         occur through passive contact, or may be enhanced through         circulation of coolant between heat sources and heat rejection         coils in contact with the skin;     -   The range extending generators may be integrated in modular bays         designed to accommodate generator alternatives, upgrade and         removal of the generators, and use of the bay to house energy         storage units instead or in addition to the generators. This may         be accomplished by sizing the bay, providing access, structural         support, and supporting infrastructure (e.g., fuel lines,         cooling, wiring, etc.). Generators bays may be positioned in one         or more of several locations:         -   Fuselage bay aft of the main cabin;         -   Wing mounted nacelles; or         -   Non-structural fairings; and     -   Propulsors are designed for upgrades to higher efficiency, or         higher power motors, which may include a new fan. Unlike         conventional engines, this is accomplished with minimal         (re)engineering.

Note that aircraft variants with performance tailored to different markets are readily enabled by the modularity of the hybrid-electric powertrain. The separation of thrust generation (by the electric propulsors) and power generation (by the hybrid-electric powertrain) enables development of aircraft variants with widely varying performance via tailoring of powertrain modules to the application, coupled in some cases with a change in the propulsors. This enables development of aircraft with widely varying performance, speeds, ranges and operating costs, based on the choice of powertrain modules and propulsors. Given the resulting limited impact on aircraft handling and maximum weights, the (re)engineering and certification required is modest. This is in contrast to conventional aircraft where variants require significant engineering and certification re-work. In some embodiments, the development of aircraft variants may occur by the following process:

-   -   Hybrid-electric aircraft variants may be developed by modifying         the baseline aircraft via a compressed aircraft design process,         focused on access, interior layout, pressurization, cockpit, and         performance. In such a case, the following steps/stages may be         used to design a variant:         -   Define interior configuration and payload requirements;         -   Define the cockpit configuration, for example, a manned             system with provisions for unmanned operation in the future.             The following types of aircraft control may be supported:         -   Fully piloted;         -   Piloted with remote backup—primary control by one or more             pilots onboard the aircraft, and equipped for secondary             control by a remote pilot;         -   Remotely piloted—equipped for primary control by a remote             pilot, with or without assistance onboard; or         -   Fully autonomous—equipped for primary flight without human             control, and may be equipped for secondary control by a             remote or onboard pilot; and     -   Specify the performance requirements for the target market         ranges and operating conditions (such as the 3-tier (A), (B)         and (C) ranges/design requirements described herein), including         variance with technology level, and optimize the powertrain to         meet these requirements using mission analysis with the         aerodynamics and propulsion of the baseline aircraft.

The following represent examples of aircraft variants that may be designed and implemented using the described methodology:

Example 1: Commercial Variant

-   -   Cabin configured for economy seating, passenger baggage         allocation in line with standard for commercial carriers.         Baggage space in interior of cabin and in hold;     -   Control system configured for minimum of single pilot with         remote pilot as backup, with option for second pilot, if         required or trainee;     -   Upper range limit is point where passenger would switch to         commercial jet travel as more time and cost efficient.         Infrequent extended range operations;     -   Market segment highly sensitive to (DOC+I), less sensitive to         COT; therefore matched with lower cost range-extending generator         (e.g., TDI) aligned with slower design speeds. Pressurization to         lower altitudes, except on variants for use on very short legs         (<200 miles); and     -   The sample speeds, ranges, and resulting powertrain         configuration shown in FIG. 34 are representative of this class         of aircraft.

Example 2: Business Variant

-   -   Cabin configured for business seating, baggage allocation above         standard for commercial carriers. Baggage space in interior of         cabin and in hold;     -   Control system configured for minimum of single pilot with         remote pilot as backup, with option for second pilot, if         required or trainee;     -   Less predictable routes, more frequent use of extended range;     -   Highly sensitive to COT, less sensitive to (DOC+I); variant may         be matched with a higher power range-extending generator (e.g.,         aircraft gas turbine) aligned with higher design speeds and         altitudes for extended range cruise; pressurization to         intermediate altitudes; and

Example 3: Cargo Variant

-   -   No pressurization, or cabin furnishings;     -   Control system configured for pilot optional flight, with         control by remote pilot on unmanned legs;     -   Speeds and ranges specified to target niche between ground         transport and commercial aircraft, typically 200-700 miles,         intermediate speeds;     -   Market segment highly sensitive to (DOC+I), least sensitive to         COT; therefore matched with lower cost range-extending generator         (e.g., TDI) aligned with slower design speeds unless required by         longer range requirements; and

As described herein, the inventive aircraft are designed for safety and fault tolerance exceeding stringent aviation requirements (FAA and EASA) via a powertrain architected for graceful degradation. This includes the ability to tolerate failures in power sources (energy storage units, generators), motors (propulsion, generator), convertors (inverters, rectifiers, DC-DC convertors), distribution (buses, wiring), controls (sensors, communication), as well as safety in the event of moderate or severe impact on the system.

The inventive aircraft and powertrain operation are designed for optimal efficiency over regional ranges; this is due in part to the flight path optimization process implemented by the FPOP and the operation of the powertrain for optimal efficiency, and may further include energy recovery through regenerative braking, and center of gravity adjustment through stored energy positioning for drag reduction. These aspects are described further in the following:

-   -   Flight efficiency is improved by flight path optimization         capabilities unique to the hybrid-electric aircraft, including         efficient flight at low altitudes, and stored-energy first         utilization. Optimization is accomplished through the Flight         Path Optimization Platform (FPOP), described herein. This         contrasts with conventional flight path optimization in which         efficiency is strongly dependent on altitude and there is little         opportunity for flight path modifications other than to fly as         high as possible;     -   Within the optimized flight path, powertrain is operated for         optimal efficiency as described herein;     -   The powertrain is designed for energy recovery via regenerative         braking of the propulsors, as described herein. Conventional         aircraft have no way to recover energy from drag producing         devices such as spoilers; and     -   The stored energy units in the fuselage may be used to adjust         the aircraft center of gravity (CG) to simplify loading, and to         reduce drag in cruise mode. The aircraft payload weight should         be distributed such that the CG is within an established         envelope close to the center of lift, and within the envelope,         aircraft drag is reduced by moving the CG aft. Being able to         relatively quickly adjust CG location allows the operator         efficiency gains by speeding the loading process, and reducing         drag;         -   CG movement with stored energy units may be accomplished by             providing a series of bays along the fuselage, and             selectively utilizing only a fore or aft location. Another             implementation has energy storage units mounted on tracks             allowing fore and aft translation as commanded by the pilot             or flight control system;         -   Conventional aircraft may have some capability to move CG             with selective utilization of different tanks in the fuel             system, but once the fuel is burned off in flight, the             benefit is reduced and typically lost.

The Table below contains certain parameters for an example of a hybrid-electric aircraft designed in accordance with the principles and processes described herein. The 4-view of FIG. 33 shows a concept 40 person/seat regional hybrid-electric aircraft designed using the inventive HEV aircraft design process. Overall size and weight are similar to the conventional ATR-42-500 (48 seats, twin engine turboprop). Given energy requirements, design of the aircraft is based on battery energy densities ranging from 600 Wh/kg to 900 Wh/kg. These enables an electric range of 170-280 nm, hybrid of 425-500+nm, at minimum cruise speed of 380 KTAS, and cruise altitudes between 18,000 and 25,000 ft.

Note that the aircraft illustrated in FIG. 33 is pictured in one possible configuration for an aircraft meeting the general requirements stated. In this example, three integrated electric ducted fan propulsors are used to provide thrust, and the position over the aft fuselage reduces drag through boundary layer ingestion and wake momentum deficit restoration. Pods at the base of the vertical tails house the gas turbine generators; the inlet and exhaust are faired over when the generators are not running to reduce drag. The subcritical cruise Mach value allows use of a light weight, straight wing, and the propulsor location on the tail allows short, lighter weight landing gear. Noise reduction may be achieved through quiet ducted fans with additional reductions due to mounting above the fuselage and between the tails, blocking much of the fan tonal noise. The weights, sizes and performance of the aircraft designed are below, including improvements enabled by future higher energy-density batteries.

Size Propulsion and powertrain Wing span 75.15 ft Powertrain type hybrid-electric Wing area 520 ft{circumflex over ( )}2 Propulsion motor 3 × 1600 SHP Overall length 62 ft Power loading 7.92 lbs/hp Overall height 20 ft Ducted fan Variable-pitch fan Cabin length 34.8 Ft Fan blades 16 Cabin height 74 In Range extending generators 2400 SHP Cabin width 98 In Generator 1800 kW cont Number of 40 Maximum cruise fuel bum 1080 pph passenger seats Pilots 2 Weights Stored energy Maximum take-off 38000 lbs Battery mass, Cell 2500 Kg weight weight Maximum useful 11500 lbs Battery mass, Pack 3000 Kg load weight Maximum fuel 2500 lbs Total stored energy, 2250 kWh capacity new Wing loading 73 PSF Total stored energy, 1912.5 kWh 1000 cycles Take-off and Landing Cruise performance Take off ground 1100 ft Stored energy 600 900 Wh/kg roll density Clear 50′ obstacle 1500 ft Energy Capacity 1500 2250  kWh Standard cruise 335 355 KTAS Max rate of climb, 2400 fpm Maximum cruise 380 380 KTAS SL Long range cruise 320 340 KTAS Stall speed, Clean 120 KIAS Hybrid range 425  500+ Nm Stall speed, Full 65 KIAS Electric range 170 280 Nm flaps Landing distance 1300 ft over 50 ft Flap system Active high lift The table indicates several of the unique aspects of the hybrid-electric design. Fuel burn and fuel capacity are less than half of the conventional equivalent. Cruise performance is given for two levels of stored energy, 600 and 900 Wh/kg; this level of improvement would be expected over 4-8 years of aircraft operating life depending on advances in energy storage technology. Lastly, the maximum cruise speed is much higher than might be expected, with propulsion motors retaining full power at altitude.

As mentioned, FIG. 34 is a diagram illustrating the efficiency of a certain aircraft and propulsor configuration as a function of flight altitude and required power. The curves illustrate how an aircraft which is energy (not power) limited will be able to cruise at successively higher speeds and altitudes as the energy limits are increased. The envelope extends from an initial cruise speed on the order of 200 KTAS with initial energy storage density on the order of 350 Wh/kg, increasing to over 260 KTAS as storage density improves to 900 Wh/kg, a 2.6× change which is expected to occur over about 10 years given current rates of energy storage technology improvement. As part of this innovation, it is recognized that this performance improvement will only be available to the operator if the higher speeds and altitudes are included as design points from the beginning of the design process (rather than limit to initial performance, as would be done with conventional propulsion).

FIG. 35 is a diagram illustrating several regional zones and the associated airports or landing areas that may be used as part of implementing an embodiment of the inventive regional air transportation system. As shown in the figure, each regional zone (e.g., “Pacific Northwest”, “Pacific Southwest”, etc.) may contain multiple landing strips and/or formalized airports (as indicated by the dots within the regions). Note that each regional zone may contain tens to hundreds of potential airports or take-off/landing sites for the inventive aircraft, and may contain a regional hub or other form of centralized location. Aspects of the control of the regional air transportation system may be located at one of several data centers or scheduling/flight monitoring facilities. Such facilities may operate to individually and/or in aggregate to schedule flights at multiple airports, generate flight plans/paths and the corresponding instructions for one or more aircraft, communicate such instructions to one or more aircraft, and monitor the flight and its flight data for one or more aircraft.

The inventive hybrid-electric air transportation system offers significantly lower door-to-door travel times and lower total costs per mile than alternate regional travel modes such as highways, rail or high-speed rail, or conventional air. This is achieved via convenient high-frequency “close-in” flights to a large number of regional airports near communities and population centers, using quiet range-optimized hybrid-electric aircraft. Additional beneficial features of the system include:

-   -   The availability of airport onsite electric energy generation         and storage. Many of the airports may be equipped with onsite         electricity generation and storage facilities to minimize         electricity costs. Onsite generation, e.g., solar, wind, etc.         may be used to recharge aircraft batteries and power the         airport, delivering excess to onsite storage or to the         electrical grid. Onsite electric storage will enable optimal         purchases of electricity from the grid (e.g., at off-peak rates)         and storage of electricity generated onsite for later use.         Retired aircraft batteries could be used in onsite storage         through late-life prior to disposal; and     -   A variety of cost-effective last-mile ground travel options at         the airports, from origin and to destination. The regional         airports may offer passengers a greater variety of ground         travels options than are offered at non-hub airports today.         Several powerful trends playing out currently will encourage         this: electric and autonomous vehicles (e.g., Tesla, Google,         Uber, Apple), ride sharing (e.g., Lyft, Uber, Sidecar,         RelayRides), fractional car rentals (ZipCar, Hertz-on-demand).         Some regional airports today are already connected to local mass         transit; over the next 5-10 years, electric and autonomous         shuttles will enable a larger fraction of airports to offer         inexpensive connectivity to mass transit. This will be         supplemented by multiple personal auto and taxi alternatives         enabled by the trends above, for example, pick-up by autonomous         cars, fractional rentals, and various forms of ride sharing.

In accordance with one embodiment of the invention, the system, apparatus, methods, elements, processes, functions, and/or operations for enabling the inventive aircraft, transportation system, and aircraft control system or transportation system control system may be wholly or partially implemented in the form of a set of instructions executed by one or more programmed computer processors such as a central processing unit (CPU) or microprocessor. Such processors may be incorporated in an apparatus, server, client or other computing or data processing device operated by, or in communication with, other components of the system. As an example, FIG. 36 is a diagram illustrating elements or components that may be present in a computer device or system 1900 configured to implement a method, process, function, or operation in accordance with an embodiment of the invention. The subsystems shown in FIG. 36 are interconnected via a system bus 1902 (as may also be one or more of the subsystems illustrated in FIGS. 21 and 22). Additional subsystems include a printer 1904, a keyboard 1906, a fixed disk 1908, and a monitor 1910, which is coupled to a display adapter 1912. Peripherals and input/output (I/O) devices, which couple to an I/O controller 1914, can be connected to the computer system by any number of means known in the art, such as a serial port 1916. For example, the serial port 1916 or an external interface 1918 can be utilized to connect the computer device 1900 to further devices and/or systems including a wide area network such as the Internet, a mouse input device, and/or a scanner. The interconnection via the system bus 1902 allows one or more processors 1920 to communicate with each subsystem and to control the execution of instructions that may be stored in a system memory 1922 and/or the fixed disk 1908, as well as the exchange of information between subsystems. The system memory 1922 and/or the fixed disk 1908 may embody a tangible computer-readable medium.

Note the following variables, parameters, and units are understood as being used in the description of embodiments of the inventive regional air transportation system.

Units Variable Stored Fuel Definition Usable energy kWh kg Usable stored electrical energy and fuel onboard the aircraft Available energy kWh kg Usable energy less Reserve ener

Required energy kWh kg Energy needed to fly a defined 

Arrival energy kWh kg Predicted Available energy after landing at th

destination Safety reserve kWh kg Energy which should remain at arrival to provide safe and legal reserves Contingency kWh kg Additional reserve to account for flight reserve uncertainty Reserve energy kWh kg Sum of Safety reserve and Contingency reserve Airspeed m/s Indicated airspeed Aircraft CG m Location of the longitudinal center of gravity from reference datum Aircraft speed m/s Inertial speed Aircraft weight kg Total weight of the aircraft Ambient pressure Pascals Atmospheric air pressure Ambient Degrees Celsius Atmospheric air temperature temperature APU Generator kW Instantaneous power output from all power generation sources Battery power kW Instantaneous power output from all battery sources Caution index integer Index to table look up of potential flight hazards (eg: turbulence) Constraint type integer Altitude constraint, options are: fixed, min, max Display mode Calibration, Flight prep, Inflight control, Diagnostics Duct exit area % Percent adjustable duct exit area reduction: 100% min, 0% max Flight duration s Time from start of take-off roll to end of landing roll Flight mode Optimal, High speed, Economy,. Fuel flow rate liters/s Rate of fuel consumption by the generators(s) Hazard Potential en route flight hazards, e.g., icing, turbulence, precipitation KCAS nm/hour Knots Calibrated Airspeed: the corrected airspeed read out to the pilot KTAS nm/hour Knots True Airspeed: inertial speed, equals ground speed in no wind Motor power kW Power delivered to the propulsor at the shaft of the electric drive motor(s) Motor regen RPM RPM Rotational speed of the motor(s) when in regeneration mode Motor regen torque Nm Torque delivered by the propulsor to shaft of the electric drive motor(s) Motor RPM RPM Motor rotational speed Motor torque Nm Torque delivered to the propulsor at shaft of the electric drive motor(s) Nautical mile or 1,852 meters Nm Pilot generator on- integer Generator state request from direct pilot off control: On or Off Pilot generator kW Generator power delivery, as requested by power pilot control Pilot power % Propulsor power output via pilot movement of the Power lever Pilot regen braking % Pilot commanded regen braking, with brake pedals (ground operation) Pilot reverse power % Pilot commanded reverse thrust (ground operation) Propeller blade pitch degrees Propeller blade angle, automatically angle adjusted Segment type integer Flight segment type: take off, climb, cruise etc. Uncertainty factor % Pilot confidence in flight plan and conditions (eg. weather forecast) Weather index integer Index to table look up of weather conditions

indicates data missing or illegible when filed

Flight track Latitude Longitude Altitude Alt min Alt max Constraint type Waypoint 1-N degrees degrees m m m Integer Flight Path Latitude Longitude Altitude Speed Segment type Waypoint 1-N degrees degrees m m/s integer Air path Distance Altitude Speed Segment type Waypoint 1-N m m m/s Integer Energy Stored Fuel Power ratio Generator plan Energy available (Storage/ power available generation) Waypoint 1-N % full kW % full kW % kW Weather Latitude Longitude Altitude Direction Speed Temperature index Waypoint 1-N degrees degrees m degrees m/s Celsius Cautions Latitude Longitude Alt min Alt max Hazard index Waypoint 1-N degrees degrees m m integer Aircraft Available Safety Contingency Weight Aircraft CG Flight Uncertainty state energy reserve reserve mode factor Current kW, kg kW, kg kW, kg kg Aircraft CG Integer %

It should be understood that the present invention as described above can be implemented in the form of control logic using computer software in a modular or integrated manner. Based on the disclosure and teachings provided herein, a person of ordinary skill in the art will know and appreciate other ways and/or methods to implement the present invention using hardware and a combination of hardware and software.

Any of the software components, processes or functions described in this application may be implemented as software code to be executed by a processor using any suitable computer language such as, for example, Java, JavaScript, C++ or Perl using, for example, conventional or object-oriented techniques. The software code may be stored as a series of instructions, or commands on a computer readable medium, such as a random access memory (RAM), a read only memory (ROM), a magnetic medium such as a hard-drive or a floppy disk, or an optical medium such as a C_(D)-ROM. Any such computer readable medium may reside on or within a single computational apparatus, and may be present on or within different computational apparatuses within a system or network.

All references, including publications, patent applications, and patents, cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and/or were set forth in its entirety herein.

The use of the terms “a” and “an” and “the” and similar referents in the specification and in the following claims are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “having,” “including,” “containing” and similar referents in the specification and in the following claims are to be construed as open-ended terms (e.g., meaning “including, but not limited to,”) unless otherwise noted. Recitation of ranges of values herein are merely indented to serve as a shorthand method of referring individually to each separate value inclusively falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate embodiments of the invention and does not pose a limitation to the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to each embodiment of the present invention.

Different arrangements of the components depicted in the drawings or described above, as well as components and steps not shown or described are possible. Similarly, some features and sub-combinations are useful and may be employed without reference to other features and sub-combinations. Embodiments of the invention have been described for illustrative and not restrictive purposes, and alternative embodiments will become apparent to readers of this patent. Accordingly, the present invention is not limited to the embodiments described above or depicted in the drawings, and various embodiments and modifications can be made without departing from the scope of the claims below.

APPENDIX C—METHODS FOR IMPLEMENTING QUIET, EFFICIENT AND LIGHTWEIGHT ELECTRICALLY-DRIVEN PROPULSION FOR REGIONAL AIRCRAFT Background

Transportation systems and transport platforms are an important part of the infrastructure used to enable commerce and the movement of people between locations. As such, they are essential services for the growth of an economy and the development of a society. Over the years, several types of transportation systems have been developed, each typically with their own focus, advantages and drawbacks.

A key challenge for transportation is the developing “gap” over regional distances, where much of the world is left without a high-speed mode. Competitive pressures have driven a 70-year decline of air over these distances. And the impact of high-speed rail is limited by economics to a few dense corridors. As a result, the last time the US travel survey was conducted in 2001, 90 percent of all long-distance trips were over regional distances from 50 to 500 miles. Yet just 2 percent of these were by air, with auto at a staggering 97 percent. This has had a tremendous impact on mobility and economic development. Regional door-to-door mobility has stagnated: travel times by highway have not improved for decades; and flight times have stretched given slower cruise speeds and increasingly congested airports. Meanwhile, the steady consolidation of air services to a declining set of major hubs has left many communities disconnected from the global air network, with severe impact on their economy and ability to attract investment.

This bleak landscape is poised for dramatic change as sharing, electrification and autonomy converge to enable fast and flexible regional transport at scale. Intra-urban travel is already being reshaped by ridesharing, car sharing, car and van pooling. Electric vehicles and driverless technologies will take this further. Lower fares, improved productivity and reduced congestion will dramatically expand utility and extend range. Crucially, as recognized by the inventors, the transformation on the ground will take to the skies, extending impact of this “new transport” to regional, and eventually, intracontinental ranges. A new breed of small- to mid-sized hybrid-electric aircraft will usher in a golden era of regional air. Frequent departures of smaller aircraft from a large number of smaller airfields will enable air travel much faster than today and at much lower fares. Over time, rapidly improving batteries will further reduce costs and extend range, while increasing acceptance of drones will gradually reduce the need for pilots onboard, accelerating the trend.

As recognized by the inventors, this scale-out of air transport via small to mid-sized aircraft flying regional ranges is a direct result of the unique operating economics of range-optimized hybrid-electric aircraft, enhanced further by future autonomy. Conventional aircraft are enormously scale and range advantaged. Larger aircraft are more efficient, as are flights over longer ranges, than smaller aircraft or shorter flights. These economies of scale and range have powered the long-term transition of aviation to large, long-haul aircraft and high-volume hubs, destroying the utility of regional air. In contrast, hybrid-electric aircraft are free of the scale and range economics that plague regional aviation today. Smaller electric aircraft fly as efficiently as larger ones. And relatively small electrics flying regional are competitive with the largest conventional jets flying long-haul. Released from the constraints of scale and range, operators will mold aircraft, frequency and routes to travel patterns. The air network that emerges will be distributed, with frequent flights to a large number of community and urban airports, a contrast to the concentrated network of today.

As also recognized by the inventors, a critical technology for scale and range free economics are quiet, highly efficient and lightweight electric propulsors optimized for regional operations. High efficiency, high power density electric motors are matched with fixed or variable pitch ducted propulsors with aerodynamics and aeroacoustics optimized for the altitudes and speeds typical of regional flights. Ducted propulsors deliver the high static thrust and low noise required for operations out of short runways close to urban centers, minimizing community impact. Variable pitch fans enable highly efficient operations across the operating range, take-off, climb and cruise, along with regenerative braking and reverse thrust.

For high propulsive efficiency and reduced weight, ring electric motors are utilized, driving an annular fan at the outer radius, or a conventional or annular fan at the inner radius, or even an open propeller. Ring motor designs offer higher power densities and greater opportunities for lightweight air cooling. They also translate to a much stiffer structure than conventional center mounted configurations, delivering higher propulsive efficiencies without the weight penalty of the stiffening otherwise required to maintain a small tip gap. In the case of a ring motor driving an annular fan at the outer radius, high quality tip gaps are maintained via bearings, mechanical, air, magnetic or other at the outer or inner radius. The lower stiffness of a ring motor driving at the inner radius is still significantly higher than that of a conventional center mounted drive. Additional benefits include significantly reduced duct and stator structures given much lower mass suspended within the duct, fan and stators less compromised by structural requirements, and simpler rotating variable pitch mechanisms contained within the radius of the ring motor.

The benefits of ducted fans for aircraft propulsion are well known; compared to an open propeller, the ducted fan provides higher thrust on takeoff, more efficient airframe integration, and lower noise. The ducted fan has seen widespread application as the bypass fan stage of the commercial jet aircraft engine, where it is tightly integrated with, and powered by the gas turbine core. However in this implementation, high efficiency is attained only in transonic flight at high altitudes with large engines (>2000 SHP). Applications outside this regime have been far more limited; the gas turbine is inefficient both at low altitudes and smaller sizes and there has been a lack of other suitable propulsion sources; reciprocating engines are too bulky to integrate within the duct, and attempts to locate the engine external to the duct suffer significant losses in power transmission.

This invention primarily concerns ducted fans optimized for regional aircraft operations, cruise altitudes from 8000-26,000 ft, speeds of 180-430 mph, and Mach number less than 0.7. Regional aircraft spend a higher fraction of the flight in taxi, take off, climb, descent, and landing operations, which requires high efficiency across a wide range of speeds and altitudes. All of these operating conditions are a poor match for conventional high bypass jet engines which are efficient only at the high Mach, high altitude cruise design point.

In addition to propulsion on typical jet aircraft, ducted fans are also used on a number of air and water vehicles for propulsion and control. Ducted fans are widely used on drones for both forward thrust and vertical lift, and on aircraft to enable vertical takeoff operations via fixed or swiveling ducts. Buoyant air vehicles (blimps) utilize ducted fans with the ability to swivel the duct to vector thrust for both propulsion and control. Ducted fans have also found use on small aircraft, personal “jet packs, hovercraft, and air boats. The invention described herein is primarily concerned with regional aircraft propulsion, but would apply equally well to aircraft in general, and these other applications.

Compared to a conventional jet engine bypass fan, a ducted fan optimized for low noise, high efficiency regional flights will operate at a much lower pressure ratio (1.04-1.25 vs. 1.4-1.8), with far fewer stators (half the number of blades, vs. 2.2 times blades), and with the stators located further behind the fan to minimize noise (2 chord widths vs. 0.5-1). Prior art (G. Rosen, U.S. Pat. No. 3,747,343) has shown that this stator configuration is required for maximum rotor-stator aerodynamic efficiency, and minimum fan noise. However, the conventional, high stator count immediately behind the fan, plays a critical structural function in maintaining stiffness between duct and the fan blades, maintaining the very small tip gap required for high efficiency. Prior art has not addressed how to achieve both the benefits of the low pressure, low stator count architecture while maintaining the very small tip gap; a structural design challenge requiring additional support members which penetrate the fan flow field resulting in aerodynamic losses, and increased structural weight.

It is also understood by those familiar with the art that high aerodynamic efficiency can be maintained over a much larger envelope of operating speeds if the fan blades are able to rotate around their longitudinal axis in order to maintain the optimum angle with respect to the incoming flow. This principle has long been applied to open propellers, and there have also been several implementations proposed for jet engine bypass fans (e.g. Merritt B. Andrews, U.S. Pat. No. 3,893,789), however two significant design challenges have prevented commercial application. In many configurations, the variable pitch mechanism would be affixed to the non-rotating structure supporting the center of the gas turbine, requiring complex mechanisms to pass blade angle change actuation across a rotating boundary adding significant mechanical complexity, e.g. M. Andrews, U.S. Pat. No. 3,893,789. In all cases, these mechanisms must be designed to operate under the high centrifugal forces at the blade root attach point; these have ultimately resulted in mechanically heavy and complex mechanisms which have proven impractical to implement in production.

BRIEF SUMMARY OF THE INVENTION

This invention is directed at a propulsion system for aircraft comprised of an electric ring motor coupled with a ducted fan. The propulsor assembly includes a low pressure fixed or variable pitch fan with a plurality of blades coaxially mounted inside a duct. The ring motor assembly provides the drive torque and consists of an electric machine including a rotor and a stator, with the rotor affixed to the fan blade tips, and the stator affixed to the duct. The ring motor and fan assembly is supported by bearings at the outer or inner radius designed for quality gap, rotor to stator and fan to duct, to ensure high aerodynamic and electromechanical efficiencies.

The fan blades may be actuated to vary the blade pitch angle to the oncoming flow in flight. Blade connections to the outer propulsion ring, and inner hub ring are made such that the blade may rotate around its longitudinal axis (fan radial direction). A mechanism within the hub including a drive motor and a system of gears rotates all fan blades simultaneously.

In one implementation, a single propulsion fan is followed by a fixed stator coaxially located within the duct. In another implementation, multiple propulsion fans are located sequentially within the duct, and which alternate in direction of rotation.

In other implementations, a ring motor is used to drive an annular or conventional fan at the inner radius, either a single propulsion fan followed by a fixed stator located within the duct, or multiple propulsion fans located sequentially within the duct, which alternate in direction of rotation. As before, a mechanism within the hub including a drive motor and a system of gears rotates all fan blades simultaneously.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 37 is an isometric view with cut-away of the fan, duct, and motor assembly.

FIG. 38 is a cut-away of the ring motor within the duct including the blade tip interface.

FIG. 39 illustrates the blade attachment at the inner hub, including the fully rotating variable pitch mechanism.

FIG. 40 is a cross section of the motor within the nacelle illustrating a minimum drag cooling system.

FIG. 41 is a cross section of a single blade defining the blade pitch angle.

FIG. 42 illustrates a blade with root and tip support, and loads acting on the fan blade.

FIG. 43 is a cut-away of an alternate bearing implementation where the fan support bearings are located at the hub on the inner radius.

FIG. 44 shows an implementation of the invention with two counter-rotating fans and without stators.

DETAILED DESCRIPTION

The rapid development of powerful, efficient and lightweight electric motors driven by electric vehicle applications paves the way to electrically-driven propulsion for aircraft. Our innovation couples variable pitch ducted fans with advanced ring electric motors for three-fold advantages when applied to regional aircraft: electric motors do not suffer from the gas turbine efficiency losses due to small size, and off-cruise operation; the variable pitch fan provides high efficiency across the wide range of speeds and altitudes in regional operations; and the ducted fan provides high static thrust and low noise for operation out of community and regional airports. One such implementation of the ducted electric propulsor for regional transportation was described by M. Knapp and A. Kumar in US patent application 2016/0236790, in which the electric propulsor utilized drive motors in the center of the duct, replacing the traditional jet engine combustor core

A significant challenge with conventional center-mounted drive configurations is maintaining a very small gap between fan blade tip and the inner wall of the duct, critical to maximizing aerodynamic efficiency. Maintaining this gap requires a system that is very stiff structurally, incurring aerodynamic and weight penalties. As described earlier, the challenge is especially pronounced for fans optimized for regional operations, given the lower count of stators, located further aft of the fan. The use of ring electric motors at the outer or inner radius directly addresses this challenge; by enabling high stiffness without increased weight or degraded aerodynamics. Furthermore, the motors enable use of a simplified rotating variable pitch mechanism located within the inner hub. And the higher power densities and opportunities for air cooling equate to a lighter assembly, simplifying overall structure.

FIG. 37 shows one implementation of the invention including the outer nacelle 10, annular duct 15, a low pressure, variable pitch ducted fan 11, with a plurality of blades, attached to both the ring motor rotating structure 22 by rotating attach points 20, and to a central hub structural ring 16. The blade root attach mechanism is geared 21, to enable pitch adjustment in flight. The forward aerodynamic center fairing 14 is affixed to the rotating hub structural ring 16. In this implementation, the aft aerodynamic fairing 13 is fixed to the stators 12 and may be attached to the center hub 16 through a rotating interface, or may simply float directly behind the hub without direct attachment. In another implementation, the center fairing 13 is also affixed to the rotating hub 16, while the stators 12 attach to a secondary ring which surrounds the aft fairing.

As shown in FIG. 37, and in greater detail in FIG. 38, the rotating structural ring 22 provides support of the motor rotor (windings or magnets) 32, and the fan blade mounting points 20. In this implementation, the rotating structure links directly to the motor rotor, however, an interface which utilizes gearing to adjust the ratio of fan and motor rpm is also within scope of this invention.

As recognized by the inventors, maintaining the gap between motor rotor and stator 38 is critical to efficient motor operation. The construction of the rotating structural ring 22 utilizes a high modulus material, such as carbon fiber composites, which at the optimal rotating speeds for a ducted fan of 2000-4000 rpm, limits radial deformation to a fraction of the gap width. In this implementation, the rotor is supported by caged roller bearings 33 with the bearing cage attached to the fixed structure 35 and the matching race formed by the rotating motor structure 22. While this implementation uses physical bearings, magnetic, or fluidic bearings (e.g. air bearings) are also within scope of the invention.

The motor stator (windings or magnets) 31 are affixed to the primary support structure 35 through the periodic cross-tie members 39. The fixed structural rings 35 provide the attach points for integrating the motor with airframe. As recognized by the inventors, the concentration of masses and loads along the rotational plane of the fan, supported by rings with high structural moments of inertia, results in a very light, structurally efficient propulsor unit.

Those familiar with the art will recognize that this implementation directly addresses one of the critical design points of a ducted fan: eliminating the aerodynamic losses due to the gap between the blade tip and the adjacent duct wall, and further that this loss mechanism is substantially more critical to the low pressure regional fan than the conventional jet engine bypass fan due to high blade tip loading, and aerodynamic losses dominated by rotor-stator viscous interaction (rather than non-isentropic flow losses). In this invention, the tip gap is effectively zero as the blade tip is attached to a rotating cylinder 20 embedded in the ring motor structure 22. In this way, very high aerodynamic efficiency of a highly tip loaded blade can be maintained without structural or aerodynamic penalties. This attachment need not extend the full chord at the blade tip 63 to obtain the benefits of zero-gap efficiency as the aft 40% of the blade carries very little aerodynamic load.

As recognized by the inventors, addressing the tip gap in this manner results in a substantial gain in structural and aerodynamic efficiency over the conventional design where the rotating blades must be held in very close relation to the fixed duct. This structural challenge in a conventional (hub supported) design is increased by the dynamic nature of the system as the duct and fan exhibit substantial differences in loads, masses, and response frequencies. In the low pressure fan in particular, the additional structure required, and placement of the structure within the primary flow path downstream of the fan, results in substantial weight and noise increases, and loss of aerodynamic efficiency. These issues are all solved by the circumferential ring fan drive motor:

-   -   The tip gap is sealed eliminating the need for very stiff         coupling between the fan and the duct.     -   The mass of the drive motor is concentrated outside of the         rotating fan where it is directly supported by lightweight         circumferential structure and bearings. This mounting system         eliminates the mass and support structure of a central drive         shaft.     -   The center body and structures are non-structural elements         optimized for aerodynamics.

The stators 12 are spaced 1.5-2.5 chords behind the blades for aerodynamic efficiency and low noise without compromise for structural load paths. In addition, with the propulsion motor located outside of the fan, there are no significant systems in the center hub areas 14,16, 13, and thus no systems access needs to penetrate the flow field. This is in contrast to a centrally located motor (electric or combustion) which needs fuel, cooling, and other accessory systems routing to and from the center body, all of which must pass through the fan flow field causing additional aerodynamic losses.

It is well known by those familiar with the art, that the ability to cool the motor, particularly the armature, is a limiting factor on power output, and further that overall system efficiency requires that this cooling results in minimal aerodynamic drag. Motors of this type may be air or liquid cooled, and while air cooling is the lightest weight solution, in this motor implementation, a significant problem is providing a source of clean air with a high rate of mass flow. Simply ingesting external airflow would result in water, ice and dust contamination of the motor.

FIG. 393 shows an implementation of a liquid cooling system conformal to the inner and outer skins of the duct for maximum heat transfer with no added drag penalty. Heat from the motor 31 is rejected to a coolant liquid through a jacket 301. The liquid then passes through a loop of tubing 34, a cross section of which is shown here. The tubing is conformal to the outer nacelle skin 302, and may also be conformal to the inner nacelle skin 307, thus rejecting heat to the free stream flow 310 and the duct flow 312, the latter providing a source of cooling air even when aircraft speed is very low, for example in ground operations. The heat transfer tubing and skin are constructed of light weight, high heat transfer material such as aluminum, and are insulated on the inner surface to prevent excessive heating of the inner nacelle. Liquid coolants including fixed phase (e.g. oil) or phase change coolants (e.g. 1,1,1,2-tetrafluoroethane) are within the scope of the invention.

It is further recognized by the inventors that the direction of the coolant fluid may be varied depending on flight condition. In particular, the leading edge 305 of nacelles is typically heated to prevent ice build-up during flight in icing conditions, at which time the coolant would flow in a clockwise direction as viewed here, to heat the leading edge for ice protection. However, in normal operations, excess heat transfer at the leading edge would cause premature transition of the boundary layer increasing drag, and the coolant flow direction is counter-clockwise as viewed in FIG. 39 to reject most of the heat along the aft surface, at which location the boundary layer is already turbulent and the adverse effects of heat addition to the flow are minimal.

The motor and plenum cavity are vented to ambient pressure through the gaps 19 between rotor 22 and duct 15. In one implementation, brush seals are used inside the front and rear slot to avoid any ingestion of air from the duct into the motor. In another implementation, a path is provided from the rear slot such that the boundary layer is partially ingested from the duct flow thereby reducing the intensity of the blade wake at the wall, which reduces the associated drag and noise as the wake propagates downstream.

FIG. 40 shows a cut away view of the central hub 14,16, 13. As is understood by those familiar with the art, the center section of a spinning propulsor exerts very little work on the flow due to low radial velocity of the fan, such that it is beneficial to have an aerodynamic fairing with a diameter of 20-35% of total blade diameter in this areas. In the conventional case, this center body encloses the combustion engine; in this invention the central fairing is almost purely aerodynamic in function and is constructed for minimum weight and drag. The only system contained within the hub is the variable pitch mechanism located within the fan blade structural hub 16. The fan blades 11 have a root shaft 405 which penetrates the hub 16 through a cylindrical bearing set 403 and the shaft is capped with a tapered gear 21. This gear is meshed with a Pitch Change Ring 431 which is seated within a recess 439 in the hub ring 16 such that the outer diameter of the ring is slightly more than the inner diameter of the hub. Rotating the Pitch Change Ring 431 causes all fan blades to be simultaneously rotated around their radial axis to effect a change in pitch angle with respect to the oncoming flow. The tapered gear 21 is also seated to the hub 16 with a set of thrust bearings to support the centrifugal loads of the blade. In this invention, all components of the mechanism are secured to, and rotate with the hub.

Blade pitch angle is defined in FIG. 41, which shows a cross section of a representative high bypass fan blade 500. The center of rotation 501 is between 35% and 55% of total chord 510 depending on the aerodynamic shape of the blade; the axis will be located along the blade center of pressure to minimize aerodynamic pitching moments around the center of rotation, which minimizes the torque load on the pitch change mechanism.

The pitch angle is defined as the angle between the plane of rotation 507 (zero angle) and the reference chord line 508 of the blade. This affects the angle at which the blade meets the oncoming flow 505, which is the net vector of the inflow velocity vector 504 and the apparent air velocity due to rotation rate 503.

FIG. 40 further shows a drive motor 432, a small, high torque electric motor, such as a stepper motor which meshes with the Pitch Change Ring 431 through a worm gear or similar. Opposite, a no-back clutch mechanism 433 also meshes with the Pitch Change Ring 431 and locks the ring in place when motion is not commanded such that torque loads are not transferred to the pitch change motor 432 when not actively changing blade pitch angle. The motor 432 and clutch 433 are mounted to a common assembly 436 spanning the hub 16 and are of equal weights to no impart asymmetric rotational vibration. The pitch mechanism is designed to provide, at minimum, a range of blades angles from 10° to 60° which encompass the normal range of operations. This range may be extended to a higher positive pitch, up to 85° to provide a “feathered” angle for a low drag configuration following an in-flight shut down and rotation stoppage of a propulsor.

The variable pitch range may also encompass a negative angle, e.g. −25° to be used as a thrust reverse mechanism with continuous rotation. This capability requires a disk solidity somewhat less than 1.0 to enable the blades to rotate through the 0° position without clash. As an alternative, albeit with slower response, reverse thrust can be accomplished through reversal of rotational direction.

The pitch change motor may be commanded through any number of wired or wireless information transmission systems, as are the feedback information regarding actual blade angle, read from a position sensor which may be located on one or more blade shafts 405 and/or the Pitch Change Ring 431. In this exemplary implementation, the command and response information is passed through a wired connection 421 plumbed through a stator 12 into the non-rotating aft fairing 13 ending at a wireless information passing point 422 which utilizes a fiber optic rotary coupler 422, 423. However, other means of transmitting information across a rotating boundary 409 such as conventional slip rings, RFID, magnetic induction or similar are also within the scope of the invention.

In this implementation, electrical current to power the pitch change motor 432 is also transferred across the rotating interface 409 through magnetic induction. In a second implementation, this electrical current is generated through an electrical ring motor-generator, mounted at the interface of the hub 16 and the non-rotating aft fairing 13, using the rotation of the fan to generate power.

FIG. 42 shows a representative fan blade for a variable pitch fan. The blade 11 is supported at the root by a cylindrical shank 405 which penetrates the center hub ring, and at the tip by a support disk 20 which is recessed in the rotating motor ring. Forces on the blade include lift 64 (out of the page), drag 65, and centrifugal force 66 from blade rotation 61. The blade rotates around the radial axis 67, as positioned by the pitch change gear 21, while centrifugal load is primarily carried at the root by a bearing plate 62 recessed into the hub, and secondarily by the tip plate 20. As recognized by the inventors, utilization of root and tip attachment points provide significant structural advantages to the design of the blade through reduced root bending and root centrifugal loading. This results in a lighter, thinner blade which is structurally and aerodynamically more efficient.

The inventors further recognize that in comparison to a conventional jet engine bypass fan, the dual supported blade in the electric ducted fan has three factors reducing centrifugal forces; lower rpm as enabled by a purpose designed electric machine (rather than slaved to a gas turbine cycle), lower blade weight as described in the previous paragraph, and also partial support of the centrifugal load through the tip mounting plate 20. This reduction in centrifugal load at the blade root translates to a lighter, simpler, more robust pitch change mechanism than is possible in a conventional bypass fan.

While the preferred implementation of the invention supports the ring motor at the rim with a physical bearing, it is recognized by the inventors that a high diameter, high rpm fan may result in high losses to support mechanisms, e.g. ball bearings. An alternate implementation for support of the fan and ring assembly from the hub is provided in FIG. 43. The hub 16 includes a flange 76 to interface with a structural static hub 74 supported by the structural stators 71. In this implementation, the rotating interface is supported by a pair of angular contact bearings 75 although numerous other formats for thrust and axial bearings (e.g. roller, self locating etc.) are also within the scope of the invention. The support structure 72 is reduced in size, now supporting only the motor stator, and the gap between rotor and support structure 73 has increased to allow for deformation under operational loads.

The stators 71 in the hub supported implementation (FIG. 43) serve the structural function of supporting the hub, which requires more stators (1-1.5× the number of blades) located closer to the fan. The stators are raked aft to decrease noise at the tip with minimal loss of stiffness. While this implementation does not exhibit the full aerodynamic and structural efficiency advantages of the exemplary implementation, it is still highly preferable to a center mounted drive motor: the mass of the propulsion motor is still external to the hub reducing the support loads on the stator, and, more importantly, and the blade tip gap is still sealed by direct interface with the propulsion drive ring.

FIG. 44 shows an alternate implementation of the invention which utilizes two complete motor-fan units located coaxially within the annular duct 115 which has been extended for the second fan. Fan 11 and motor 31 are as previously described, while fan 111 and corresponding motor 131 are installed to spin the opposing direction thus cancelling the swirl losses and eliminating the need for stators. While most aspects of the counter-rotating fans implementation are very similar or identical to the single fan described herein, there are a few notable differences:

-   -   The forward center body 14 is mounted to the front hub. The         middle center body fairing 114 may be attached to the front or         rear hub, and as such may spin in either direction. The break         may also occur at a slip interface at any point in between the         fans. The aft center fairing 13 is now affixed to the aft fan         hub 116 and spins with the aft fan 111.     -   There are no stators through which to pass wires for command and         feedback signals; signals are instead passed by wireless         connection from a static location on the duct 115 to a receiver         on the hub, and this may involve optical coupling, RFID, or         other wireless means of passing information.     -   There are no stators through which to pass wiring to power the         pitch change motors and clutches. Instead, the electrical power         required is generated by a small ring motor-generator (not         shown) at the interface between center shaft 114 and forward hub         16

In another implementation, the ring motor is mounted in the center fairing 74 (FIG. 43), driving a conventional fan through the full radius flange 76 attached to the hub 16 including the rotating variable pitch mechanisms. This implementation reduces the air drag and support friction losses of the outer radius ring motor, but requires a more conventional system of support through stators 71 and/or structural pass-through members between the duct and the center body. Cooling for the motor may be through air induction, or with a liquid cooling system which rejects heat through the innovation shown in FIG. 39. This implementation is not limited to a ducted fan as the central ring motor may be used to drive an un-ducted fan, a conventional propeller, or a pair of counter-rotating open rotors.

In addition to the advantages in highly efficient regional propulsion as described in this patent, the electrically driven ducted fan has substantial safety benefits over conventional gas turbines. Eliminating the gas turbine core nearly eliminates risk of turbine rotor burst, fire, and compressor stall-surge malfunction, all of which are liabilities to gas turbine operation. As previously recognized by the inventors, it is also possible to considerably enhance system safety beyond simply substituting a combustion engine for an electric machine. This includes health monitoring of electrical circuits for early fault detection, and graceful degradation of capability under failure via redundant systems, polyphase windings, time limited over-power capability to remaining electric machine capability. Many of these safety features and implementation at the system level were described in more detail in by M. Knapp and A. Kumar in US patent application 2016/0236790.

It is further recognized by the inventors that the electric drive motor and the ducted fan both possess characteristics which affect efficiency in flight, with the common parameter between the systems being the rotational speed (rpm). From the known factors which affect overall system efficiency, the optimal rpm for any given thrust setting is a coupled optimization problem, much of which can determined in advance of flight and programmed into the propulsor control system.

When the pilot, or autopilot commands the total thrust level desired from the propulsor, an additional system for propulsion optimization then determines a real-time control strategy for the propulsors based on power requested, airspeed and environmental conditions, translating power requested to propulsor rpm and pitch angles for optimal system efficiency (motor and aerodynamic efficiency). The optimal settings are then used to direct the lower-level module controllers (e.g., variable pitch fan controller, and motor controller). The optimal settings may be further modified via a fine control loop for improved performance. This may include a peak seeking loop to fine-tune the operating point, and a smoothing loop to moderate abrupt changes in settings over intervals determined by ride comfort, aircraft structural or powertrain performance constraints;

-   -   Settings optimized may include the attitude of the propulsors,         e.g., variable pitch fan angles, position of the exhaust plugs,         and the output of the propulsor motor-inverters, e.g., torque,         speed. The optimizer regulates propulsor setting over the range         of aircraft operations including standard thrust, neutral         thrust, regenerative braking, reverse thrust and recovery         thrust;     -   For standard thrust control, the optimizer is driven by         requested power for each propulsor, airspeed and environmental         conditions, and determines propulsor settings that maximize the         thrust delivered. This is done by performing a staged or coupled         optimization across the Motor and the Propulsor performance         models. In the staged approach, motor and propulsor settings are         optimized in turn. For instance, motor settings to maximize         efficiency may be determined first, by optimization using the         Motor performance model within operating constraints defined for         the motor. Following this, the propulsor attitude is determined,         e.g., fan pitch angle or position of exhaust plug, to maximize         propulsor thrust by optimization using the Propulsor performance         model within operating constraints defined for propulsor         attitude. These settings are then used to direct the operation         of the lower level controllers via the Module Interfaces within         POCS;     -   In some implementations, a look-up table may be used to         determine near-optimal values, followed by an optional         optimization step to refine estimates along the lines of those         described above;

While this invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the spirit and scope of the invention. In addition, modifications may be made to adapt a particular situation or material to the teachings of the invention, without departing from the essential scope thereof.

It will be understood that the dimensions and proportional and structural relationships shown in the drawings are illustrated by way of example only and these illustrations are not taken as the actual dimensions, or proportional structural relationships used in the implementation of the present inventions. 

1. A hybrid-electric aircraft, comprising: at least 100 seats in a single-aisle configuration; and a series hybrid powertrain to power the hybrid-electric aircraft for medium-haul flight having a range of at least 1,000 miles, the series hybrid powertrain comprising: a plurality of energy storage units having an onboard stored energy of at least 1,500 kWh; a distribution bus coupled to the plurality of energy storage units; at least one range extending generator coupled to the distribution bus to generate onboard generated power of at least 7.5 MW; and at least three electric propulsors, coupled to the distribution bus, each electric propulsor of the at least three electric propulsor producing a thrust of at least 3.3 MW to fly the hybrid-electric aircraft with a cruise speed of at least 0.7 Mach and a fan pressure ratio of between 1.15 and 1.19 at the cruise speed of at least 0.7 Mach, each of the at least three electric propulsors producing a maximum thrust of at least 4.3 MW; wherein: a degree of hybridization for the series hybrid powertrain, given by a ratio of energy expended from the plurality of energy storage units to a total amount of energy consumed by the at least three electric propulsors during the medium-haul flight having the range of at least 1,000 miles, is at least 25%; and carbon dioxide equivalent (CO₂e) well-to-wake greenhouse gas (GHG) emissions of the series hybrid powertrain are less than 0.25 lbs/Available Seat Mile (ASM).
 2. The hybrid-electric aircraft of claim 1, wherein each electric propulsor from the plurality of electric propulsors includes a variable pitch ducted fan.
 3. The hybrid-electric aircraft of claim 1, wherein the plurality of energy storage units have a specific energy of at least 250 Wh/kg.
 4. The hybrid-electric aircraft of claim 1, wherein a maximum weight of the aircraft is less than 100,000 pounds.
 5. The hybrid-electric aircraft of claim 4, wherein an energy storage mass fraction of the aircraft is at least 21%.
 6. The hybrid-electric aircraft of claim 1, wherein the at least one range extending generator coupled to the distribution bus includes at least two range extending generators to generate the onboard generated power of at least 7.5 MW.
 7. The hybrid-electric aircraft of claim 2, wherein the plurality of energy storage units are divided into at least three banks of energy storage units, each bank of energy storage units having an energy storage capacity of at least 1,500 kWh.
 8. (canceled)
 9. A hybrid-electric aircraft, comprising: at least 100 seats in a single-aisle configuration; and a series hybrid powertrain to power the hybrid-electric aircraft for a medium-haul flight having a range of at least 1,000 miles, the series hybrid powertrain comprising: a plurality of energy storage units having an energy storage capacity of at least 1,500 kWh; a distribution bus coupled to the plurality of energy storage units; at least one range extending generator coupled to the distribution bus to generate onboard generated power of at least 7.5 MW; and a plurality of electric propulsors, coupled to the distribution bus to produce at least 10 MW of thrust to fly the hybrid-electric aircraft with a cruise speed of at least 0.7 Mach, wherein a degree of hybridization of the hybrid-electric aircraft, given by a ratio of energy expended from the plurality of energy storage units to a total amount of energy consumed by the plurality of electric propulsors, is at least 25% for the medium-haul flight having the range of at least 1,000 miles.
 10. The hybrid-electric aircraft of claim 9, wherein a fan pressure ratio of the plurality of electric propulsors at the cruise speed of at least 0.7 Mach is between 1.15 and 1.19.
 11. The hybrid-electric aircraft of claim 10, wherein each electric propulsor from the plurality of electric propulsors includes a variable pitch ducted fan.
 12. The hybrid-electric aircraft of claim 9, wherein carbon dioxide equivalent (CO₂e) well-to-wake greenhouse gas (GHG) emissions of the series hybrid powertrain are less than 0.25 lbs/Available Seat Mile (ASM).
 13. The hybrid-electric aircraft of claim 9, wherein the plurality of energy storage units have a specific energy of at least 250 Wh/kg.
 14. The hybrid-electric aircraft of claim 13, wherein the specific energy of the plurality of energy storage units is at least 300 Wh/kg.
 15. The hybrid-electric aircraft of claim 9, wherein the plurality of energy storage units have a specific energy of at least 250 Wh/kg and the hybrid-electric aircraft has a range of at least 1500 miles.
 16. The hybrid-electric aircraft of claim 9, wherein each electric propulsor from the plurality of electric propulsors has a thrust output of at least 4 MW when the hybrid-electric aircraft is moving at the cruise speed.
 17. The hybrid-electric aircraft of claim 9, wherein the plurality of electric propulsions collectively have a thrust output of at least 12 MW when the hybrid-electric aircraft is moving at the cruise speed.
 18. The hybrid-electric aircraft of claim 9, further comprising a fuel storage tank, the hybrid-electric aircraft having an energy storage mass fraction of greater than 17% when the fuel storage tank is filled with fuel.
 19. The hybrid-electric aircraft of claim 9, wherein the plurality of energy storage units are divided into at least three banks of energy storage units, each bank of energy storage units having an energy storage capacity of at least 1,500 kWh.
 20. The hybrid-electric aircraft of claim 9, wherein the plurality of electric propulsions produce the at least 15 MW of thrust and have a fan pressure ratio between 1.15 and 1.19 at a cruise altitude of less than 32,000 feet while the hybrid-electric aircraft is moving at the cruise speed.
 21. (canceled)
 22. The hybrid-electric aircraft of claim 9, wherein the hybrid-electric aircraft is a vertical and/or short take-off and landing (VTOL/VSTOL) hybrid-electric aircraft. 23.-33. (canceled) 